Revealed: IBM Model M3 & M6 laptop buckling sleeves keyboards

In July 2021, I released an article that explored IBM's buckling sleeves keyboards in the form of the Model M4 and M4-1 Space Saver Keyboards for the first time in-depth. Today, we're doing something similar all over again. Again bringing often overlooked and in my opinion criminally underrated members of the Model M family into the limelight. The Model M4 family was not the first or the last keyboards to use these buckling sleeves, so let's take a look at the portable computers that spawned these switches and later brought them to the masses, helping to cement the typing prowess of IBM's early laptops in the process. This is the definitive article regarding the Model M3, Model M6 and Model M6-1 buckling sleeves keyboard assemblies. Enjoy!

Contents

A recap from the last time

In my last article on this subject, Revealed: The story of the IBM Model M4 family, I explored the IBM Models M4 and M4-1 keyboards and keypads as the title suggests. We looked at what they were, where their origins lie, what made them unique, what possible variations exist, and what they led to. These keyboards were the IBM Space Saver Keyboards (not to be confused with the IBM Space Saving Keyboard, aka, the SSK) and use the switches we've come to refer to as IBM buckling rubber sleeves or simply 'buckling sleeves' if you prefer. What made these keyboards interesting to me was their origin story as I was fascinated by the fact they were adaptations of an IBM portable computer keyboard for desktop use. As a long-time user of IBM's portable computers, I knew this meant real pedigree. After spending time with my Unicomp-made Model M4-1 for almost a year by the time of that article's publication, I came to regard the keyboard as very underrated indeed and my choice keyboard for when I need to type quietly.

As a recap of the important points from that article:

This article is very much complementary and picks up and expands on the M4's origin story in portable computers. I know the order these articles were released based on the model designation is achronological, but I felt M4 was the overall more solid ground to begin our exploration into this Model M subfamily with since they're more well known and documented examples of IBM buckling sleeves keyboards.

The M3 and M6

Model M3 (& M4/M4-1) type assembly

As shown in detail in the M4 article, this constitutes the original flavour of IBM's buckling sleeves keyboard available in exclusively 3mm key-travel form. Launched on 26th March 1991, the IBM Personal System/2 L40SX and its numeric keypad option spawned this family of keyboards. The numeric keypad bore the designation of Model M3, which I'm using to refer to any non M6/M6-1 type assembly. The L40SX was essentially the successor to the IBM 5140 PC Convertible and sported a 20MHz Intel 80386SX (hence the "SX" in the name), 2-18MB RAM, 60MB hard drive and a 10" LCD VGA (640x480) display, and has since become known for being the laptop to use IBM buckling sleeves.

Computerworld's 1st April 1991 issue shares some insight into the early work on the L40SX. Development for the laptop started in February 1990 with IBM already late to market and was specifically looking at ways to cut down development time. Founded a few months later was IBM Information Products Corporation in Lexington, Kentucky, who developed the keyboard under CEO Tony Hancock and directly pushed the design to production, skipping the usual mock-up and prototype stages. Leap Technologies of Otsego, Michigan was given the task of producing the plastics for these keyboards, who later merged with Riviera Plastic Products Company in August 1992. During the same week as the L40SX's launch, IBM Information Products Corporation was spun off to form Lexmark International, which subsequently continued to work with IBM in producing L40SX and would become the main producer of buckling sleeves keyboards for IBM for the next 5 years.

The L40SX was followed up by the IBM PS/2 CL57SX a year later, adding a colour screen and an integrated trackball. Both machines were then succeeded by the IBM PS/2 Note and PS/Note series, some of which use modified M3 keyboards with a layout more closely resembling the later ThinkPads. The L40SX keyboard assembly was also reused on the desktop space in the form of the aforementioned M4 and M4-1 Space Saver Keyboards.

The M4s were used in a variety of roles such as server console, sysadmin and docked laptop setups, and were supposed to be quiet and smaller alternatives to IBM's flagship buckling springs Model Ms of the era (which were comparatively colossal and thunderously loud). The standard M4 represents a keyboard that is simply an L40SX keyboard with lock-lights and an AT compatible controller, whereas the M4-1 adds a TrackPoint II pointing stick and supporting mouse buttons to the design. They were primarily made by Lexmark between 1993 and 1996, with Key Tronic taking over immediate production after Lexmark exited the keyboard market in April 1996 and then Unicomp taking over long term production by the end of the 1990s. Unicomp continued producing M4s until 2010 under the market name Unicomp Mighty Mouse.

The switch mechanism of the M3/M4 type is very distinctive, being a rod-actuated buckling rubber sleeve-supported membrane design. They superficially resemble Mitsumi KPQ-type switches and are technically rated to 10 million key presses, but note that many examples of IBM buckling sleeves keyboards continue to operate despite the 2 to 3 decades of time between manufacturing and today. Typical rubber dome over membrane switches require the rubber component to pull a triple duty of pressing down on the membrane, providing tactile feedback and providing a return force to allow the keycap to return up to the user after depressing. M3/M4 type switches are designed to eliminate the first duty by offloading direct actuation to a rod on the keycap that descends through the switch barrel and provides a solid bottoming-out feel. The switch feel is also distinctively different in general thanks to the form of the rubber component - it's an outward expanding sleeve that buckles straight down instead of a dome that collapses inwards.

The keycap clips onto the barrel plate with two hooks on either side of the keycap. This keeps the keycap securely on the keyboard and prevents it from rotating around. There is also an additional rod on the keycap that seems to be used for preventing overtravel to protect the innards of the keyboard.

Speaking of the innards, the M3/M4 type assembly is a sandwich of components very similar to that of typical Model Ms. There's a barrel plate on top with barrels to hold switch components at each key position, a membrane in the middle to register keypresses and a metal backplate that the barrel plate is secured onto. Unlike the plastic rivets you may expect from other Model Ms, all keyboard-sized IBM buckling sleeves assemblies have these 'teeth' riveting that seems much more robust than the plastic rivets of their larger brethren. These teeth also allow the backplate to be unhitched from the rest of the assembly, allowing for easy access to the membrane.

M6/M6-1 type assembly

In late 1992, IBM and Lexmark introduced a successor to the Model M3 on the IBM ThinkPad 700 series and Japanese-only IBM PS/55 Note C52. The Model M6 was an evolutionary change, with just the overall keyboard layout and how the keycaps mount changed. The core sleeve element is basically unchanged - in testing, I found swapping sleeves between M3s, M4s and M6s don't alter the key feel in any significant way despite the fact some sleeves may appear thinner than others. The slightly later Model M6-1 was introduced in September 1993 with the IBM ThinkPad 750 series and represented an even lesser evolutionary change for the switches themselves. As with the previous type of assembly, Key Tronic took over production of these after Lexmark's exit from the keyboard business, however, Unicomp never followed up after Key Tronic in this case. The one thing to keep in mind is that not all ThinkPads from the 1990s used M6s - if a given ThinkPad model is not mentioned to use buckling sleeves in this article, assume that it doesn't have a buckling sleeves keyboard assembly design.

Diving into the changes, the layout is the most obvious one. Apart from a few exceptions, M6s reuse the basic layout of the non-sleeves based IBM PS/2 Note N51SX (which was announced several months ahead of the ThinkPad 700) instead of sticking to what the early M3s and M4s used. The N51SX gave birth to what we now refer to as the classic ThinkPad layout, which was used and remained relatively unchanged until 2012 when Lenovo introduced the island-style AccuType keyboards onto the entire ThinkPad line-up. The keycap shape remains unchanged for alphanumeric keycaps compared to those on M3s and M4s, however, most M6s and M6-1s abandoned the squarer keycap profile for escape, function and navigation (etc.) keys. The square small key profile used on M3s and M4s usually makes identifying them from the surface very easy, thus lacking this, it's much harder to tell M6s from the average scissor-switch or rubber peg ThinkPad keyboard of the period.

The second change is the keycap mounting. Gone are the retainer clips from the earlier design, with fixed barrel sliders now being employed to keep keycaps on. This moves the switch design from a buckling rubber sleeve-supported rod-actuated membrane to a buckling rubber sleeves-supported slider over membrane design. To be honest, this technicality doesn't change a lot in typical use - the slider still provides a solid interface for actuation and aforementioned the sleeves themselves are unchanged, meaning it still has the same benefits from a design point of view. The one thing it does change is that keycaps are now much easier to remove with the crosspoint stem being the only thing keeping them attached. This can be a pro or con based on your viewpoint - it makes removing keycaps a lot quicker if, for example, you need to clean the keyboard, however, accidental detachments can be a problem.

The third change is variation in sleeve size. The aforementioned change in smaller keycaps coupled with some specific smaller form-factor variants brought about these changes, resulting in three gauges of sleeves:

Assembly-wise, much is the same as M3/M4. The M6s still use a three-layer sandwich of components. In fact, the backplate riveting distinctly reveals the shared DNA between the two keyboard types.

And finally, the differences between M6 and M6-1 are very minor but can be identified. Firstly, M6 uses brown-coloured sliders with a more primitive (or 'skeleton-like') mount socket, although the actual keycap mount fit is the same. As shown in the last photo showing sleeves, M6-1 uses black-coloured sliders but with either the same skeletal or a more filled-out socket - earlier Lexmark M6-1s were more likely to have skeletal sockets whereas later Key Tronic M6-1s were more likely to have filled-out sockets. Secondly, M6 keyboard assemblies were screw-mounted onto a keyboard frame/stiffener that wraps underneath the main battery, whereas M6-1s were mostly hinged modules that could be opened like a car bonnet (or hood, in US English) to access the host laptop's internal components. Thirdly, most M6-1s had an additional soft black cover over the metal backplate with typically only one tooth rivet revealed. But of course, nuances and exceptions can exist and will be explained in any such instances.

Laptops using buckling sleeves

As far as I'm aware, this is the first major work attempted on this subject (Model M buckling sleeves in laptops), so I've spent many moons doing my own original research in an effort to compile an exhaustive list of which portable computers used an IBM buckling sleeves keyboard. At least when I started researching, photos that could quickly reveal whether a given keyboard used IBM buckling sleeves or confirm the Model M designation were relatively scarce beyond the most common types or what I already had on hand. As time went on, I was largely able to find the photos I need thanks to those mentioned in the acknowledgements. However, I still had to make some educated guesses based on observing the known properties of other IBM buckling sleeves keyboards. I've detailed my acid test of sorts below, which is also a good TLDR on what an IBM buckling sleeves keyboard looks like. Ideally, I sought out keyboards based on a mixture of all possible properties.

Quick links

IBM PS/2 L40SX (1991-03)

Summary: Model M3, self-produced by IBM, standard sleeve gauge

The IBM PS/2 L40SX is the origin of the IBM buckling sleeves keyboard and the common ancestor of all the other keyboard assemblies in this list, the Model M3 numeric keypad and Models M4 and M4-1 discrete keyboard and numeric keypad. The keyboard assembly establishes the characteristic keycap profile, layout, backplate and riveting style and (mostly) labelling expected from Model M3 and M4 keyboards and keypads. Being a beige machine, IBM opted to use high-quality dye sublimation as the keycap text printing method, which is a rarity in this list and for portable computers since 1990.

IBM PS/2 CL57SX (1992-02)

Summary: Model M3, self-produced by IBM, standard sleeve gauge

The IBM PS/2 CL57SX is a follow up to the L40SX and (evidently) has a modified version of its keyboard. The addition of a numeric keypad overlay on the keyboard itself is the most obvious difference besides the colour, but this is a functional (software/controller) difference, not a hardware difference (I'm reasonably sure the membrane layout itself is the same). Also worth mentioning is that the CL57SX also establishes the gold accent legends common to many of the black portable computers seen below.

IBM PS/55 Note N27sx (1992-02)

Summary: Model M3, standard sleeve gauge

The IBM PS/55 Note N27sx (5527-U08) is the Japanese market version of the CL57SX that uses the same keyboard assembly albeit modified into a JIS layout. To support a full-size JIS enter and two 1u keys in place of the typically 1.5u backspace (for ANSI and ISO keyboards), 0.5 units of space was added to the right side of the alphanumeric section of the keyboard to increase the size of all the keys on the right to the standard sizes. This, however, came at the expense of the navigation keys directly right of the alphanumeric section, which are now smaller.

IBM PS/Note 2141-x82 series (1992)

Summary: Model M3 based, produced by Lexmark, standard sleeve gauge

The IBM PS/Note 2141-x82 series were 386 based notebooks from 1992 that seem to feature a 7-row M3/M4 type keyboard. Photos and information on this series are very limited, but several variants exist including the 182, E82, N82, S82 and W82 and some photos that reveal the nature of the keyboard have been found. WorthPoint thankfully has some archived eBay listings that give a close enough view to tell these use keycaps with the characteristic M3/M4 profile. A photo found on Allegro Lokalnie also shows a brief view of the keyboard's backplate that has the expected teeth riveting and a Lexmark Model M-style sticker. Unfortunately, the only photo available doesn't reveal the Model M designation this bares, but there's sufficient evidence to say this has a variant of the Model M3. The keyboards have a 33Gxxxx part number nomenclature.

IBM PS/55 Note T22sx (1992)

Summary: possible Model M4 based

The IBM PS/55 Note (or ThinkPad) T22sx was IBM's first tablet notebook released in Japan, featuring a 7.6" monochrome touchscreen. Visually, the tablet is recognisable for having a numeric keypad with rounded buttons built into the machine. A PS/2 keyboard attachment was an optional extra and translation of Japanese text seems to indicate IBM called it a "sheet-type keyboard". Anyway, the arrow keys have the distinct M3/M4 style keycap profile although what is interesting about the T22sx keyboard is that the entire keyboard uses the smaller-type keycaps that are typically limited to just the navigation and F-keys on Model M3 and M4 keyboards. Well, non-alphanumeric, non-modifier and non-arrow keys on this keyboard are half-depth-unit instead. Photos are limited, but komotch2 provides the most detailed photos of this keyboard (especially from views other than the top) and states its part number to be 48G9868. I imagine the keyboard's electronics are similar to the Model M4's as well, with just the SDL connector being replaced with a fixed cable, but this is just my speculation.

Lexmark Lexbook AR-10 & GS-20 (1992-04)

Summary: Model M6 based, produced by Lexmark, standard sleeve gauge

The AR-10 (with 16mm trackball) and GS-20 (with pointing stick) were two similarly sized Lexmark OEM laptops existing as early as April 1992. Just like how Lexmark produced buckling springs Model M keyboards for other companies such as Better On-line Solutions, Dell or Lynx, Lexmark seemed to have done the same for laptops. What's interesting about these is that they seem to use a modified Model M3 keyboard, perhaps derived from the IBM PS/Note 2141-x82 series' keyboard, however brown M6 barrel sliders are observed. The design as seen sports a choice of pointing device with two (presumably) buckling sleeves mouse buttons on the keyboard itself - the "M1" and "M2" buttons in between Ctrl and Alt. According to a Tech Monitor article from March 1994, it seems Lexmark-self branded examples (called Lexbooks) didn't appear until 1994. So, third-party branded versions actually came before. The three examples I was able to find of an AR-10 were the CompuAdd Express 325 series (325NXL seen on WorthPoint and reviewed PC Magazine in August 1992, 325FTX seen on WorthPoint and Reddit), CompuAdd 425FTX, Cube ProBook 425NTX (found in an InfoWorld magazine issue from 24th May 1993) and Hyundai Courier Spectra (found in a PC Magazine issue from August 1993), both 486SLC based machines. The only photos I can find of a Lexmark-branded one was on WorthPoint. I've yet to see a GS-20 based machine outside of Lexmark's own adverts.

Update 2022-09-26: I've since acquired a CompuAdd 425FTX, which is currently with my friend - TheMK. They've kindly taken some photos of it that reveal so much more about the AR-10 keyboard assembly. They also confirm that the mouse buttons are buckling sleeves driven.

Update 2023-02-02: I now have the aforementioned laptop in my possession. The buckling sleeve mouse buttons have noticably less travel and feel more 'binary'. For future work, I'm now designating these as "mouse button type buckling sleeves".

IBM ThinkPad 700 series & PS/55 Note C52 (1992-10)

Summary: Model M6, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 700 series were 486SLC based laptops that constituted the first 'true' ThinkPad laptops released by IBM in October 1992. Comprised of the 700 and 700C (the difference being largely the screen), these were the first laptops to use true TrackPoint (revision II) strain gauge pointing sticks and one of two series of ThinkPads to use a Micro Channel Architecture (MCA) bus. The PS/55 Note C52 was a Japanese only computer that was largely identical to the 700, including the keyboard. Along with the later 720 series, the 700 series uses 44G379x (English speaking) and 48G92xx (rest of world) nomenclature Model M6 keyboard assemblies as per IBM Mobile Systems Hardware Maintenance Manual Volume 2. The PS/55 Note C52 specifically uses a P/N 44G3796 Model M6. These keyboards in fact represent the first in the long line of Model M6/M6-1 keyboards that are barrel slider actuated instead of keycap rod actuated, with these using the first brown-coloured slider design. An interesting side note is that the German models of these ThinkPads were beige, with their P/N 48G9236 using IBM's famous pearl (off-white) and pebble (grey) keycap colour scheme and also seems to have dye-sublimated keycaps like the L40SX as well (just excluding the arrow keys, though).

IBM ThinkPad 720 series (1993-05)

Summary: Model M6, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 720 series were 486SLC2 based laptops that followed the original 700 series. Once again, a non-C and C model was released and the German model was available in a beige colour, but these were the last ThinkPads to feature MCA. IBM Mobile Systems Hardware Maintenance Manual Volume 2 confirms that the 720 series uses the same TrackPoint II-based 44G379x (English speaking) and 48G92xx (rest of world) nomenclature Model M6 keyboard assemblies as the earlier PS/55 Note C52 and 700 series.

IBM ThinkPad 350 series & PS/Note 425 series (1993-06)

Summary: Model M6, produced by Lexmark, TrackPoint II, standard sleeve gauge

The IBM ThinkPad 350 series were 486SL based laptops that succeeded the original ThinkPad 300, consisting of the 350 with a monochrome screen and the 350C with a 256-colour screen. Unlike the 300 that was produced for IBM by Zenith Data Systems, the 350 is in-house built and features Lexmark-produced buckling sleeves keyboards. The PS/Note 425 series was an identical sister system to the 350s and were the last of the PS/Note family. Both systems use the same keyboards with 60Gxxxx part number nomenclature as per IBM Mobile Systems Hardware Maintenance Manual Volume 1. Photos of the back of 350 keyboards show a Lexmark sticker without a Model M designation, however, photos of 425 keyboards show a Model M6 designation. This is puzzling since the F and navigation keys use M3/M4 style keycaps, but indeed, these have characteristic brown barrel sliders like other M6s.

IBM ThinkPad 500 (1993-06)

Summary: Model M6 based, TrackPoint II, medium & small sleeve gauges

The IBM ThinkPad 500 was a small 486SLC2 laptop that can be identified by its unusual placement of left and right mouse buttons on the front of the laptop. Its keyboard assembly for the most part features M3/M4 profile keycaps with the exceptions being the numerous half-depth keys on the top row and the bottom-right navigation cluster, something highly unique for buckling sleeves keyboards. Additionally, photos of the backplate confirm the presence of M3/M4/M6 riveting and Jack @ laptop.pics confirm it indeed feels like a buckling sleeves keyboard. The keycap legends colour is CL57SX style. Its keyboard part number is 59G7920. The one remaining question is whether this is a late M6 or an early M6-1, but given the 500 was announced at the same time as the ThinkPad 350, it's unlikely that they would use different variants of the M6.

IBM ThinkPad 750 series (1993-09)

Summary: Model M6-1, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 750 series were 486 based machines first released in September 1993 and are known for being the first ThinkPads in space. September saw the launch of the 750, 750C and 750P, with 750CS joining the family in November 1993 and 750CE in June 1994. 750s likely have the distinction of bearing the first Model M6-1s with black barrel sliders instead of brown, using 66G0xxx nomenclature keyboard assemblies as per IBM Mobile Systems Hardware Maintenance Manual Volume 2. These also introduced the feature where the keyboard would act as a lid for accessing the machine's internals, and the ThinkPads 370C, 755C and 755Cs would later reuse these assemblies. In regards to their use within 750s, these assemblies seemed to have been exclusively produced by Lexmark. Like the previous 7xx series ThinkPads, the 750 also had a beige counterpart in Germany.

Lexmark Lexbook MB-10 & MB-15 (1993-12)

Summary: Model M6 based, produced by Lexmark, medium & small sleeve gauges

The Lexmark Lexbook MB-10 and MB-15 were a pair of MS-DOS subnotebooks with Chips and Technology F8680 packages (a sort of early system-on-a-chip CPU and GPU combo with direct PCMCIA access) and carried MS-DOS 5.00 in ROM. They two were advertised as early as December 1993 in PC Magazine. Weighing about 1kg, these are some of the smallest buckling sleeve bearers in this list. The difference between MB-10 and MB-15 seem to be up for debate but possibly has to do with RAM amount. You may think this very compacted keyboard looks very familiar, and you'd be right - it seems to be based on the keyboard assembly of the IBM ThinkPad 500 and 510CS (the latter of which is a little further down in this list). The tiny keys on the top row are a dead giveaway, although there's now some spacing between the keys as the F11 and F12 keys have been deleted from the design. The bottom row has also been completely overhauled, mostly for the better. The tiny arrow keys are gone, with full-size ones now in their place (the other navigation keys are now on the function layer on top of these arrow keys). The design also now includes a "mouse-key" instead of a TrackPoint II pointing stick, which some say acts like a little joystick. If I had to guess, it's probably an FSR based stick. Like the AR-10 and GS-20, two mouse buttons are found in the bottom-left corner.

IBM PS/55 5535-ZAD (1994)

Summary: Model M3 based, presumably produced by Lexmark, presumed TrackPoint II, standard sleeve gauge

The IBM PS/55 5535-ZAD is a large ThinkPad-style computer for the Japanese market believed to have been released in 1994, making it a fairly late M3/M4 style buckling sleeves bearer. Photos of this machine are very scarce with aichi JAPAN being the only readily accessible repository of photos, but the available photos show M3/M4 keycap profile and M3/M4/M6 riveting. The 5535-ZAD keyboard assembly is interesting in that it's the most 'standard' Model M3/M4 style keyboard released, being a standard tenkeyless JIS layout with standard unit-size modifier keys unseen on other Model M3s and M4s. It also has CL57SX style keycap legends colour and a second backplate with what appears to be heat treated to give it a rainbow-like colour pattern. A hole in the second backplate near the middle reveals a single viewable M3/M4/M6 style rivet.

SAIC Galaxy 1100 (1994)

Summary: Model M3 based, presumably produced by Lexmark, standard sleeve gauge

The SAIC Galaxy 1100 was a portable workstation based on the HP 9000/712 series and designed for military and intelligence usage. It used a PA-RISC based PA-7100LC processor at 60 or 80MHz, had 128MB RAM and was built to several military and government standards. This second behemoth of 1994 seems to use a Model M3 keyboard very similar to the one found on the IBM PS/2 L40SX except it features a red numeric keypad overlay and has LED lock-lights. Since the Galaxy 1100 is very rare, I don't expect to find out if it has an internal birth certificate sticker and Model M designation. But I guess that it would be M3 or maybe M4 due to the lock-lights.

Tadpole SPARCbook 3/SPARCbook 3000/P1000/ALPHAbook 1 & IBM RS/6000 Notebook N40 (1994-02)

Summary: Model M6-1, produced by Lexmark & Key Tronic, TrackPoint II, standard & medium sleeve gauges

The Tadpole SPARCbook 3 was as the name implies a SPARC (a RISC-based processor) based laptop family that included at least the SPARCbook 3, 3GS, 3GX, 3LC and 3TX. Tadpole is known for making military spec machines and for using many types of processor architectures. The Centre for Computing History says these use an "IBM Corp Thinkpad keyboard with an integrated Pointing Stick", and on further inspection, these indeed seem to use keyboard assemblies manufactured by Lexmark. A spec sheet hosted by MCbx Old Computer Collection states the 3TX in particular to use a "Lexmark 84-key" keyboard. In the third photo shown above, you can also see the characteristic IBM buckling sleeves style riveting on the keyboard's backplate. The SPARCbook 3 seems to have spawned several derivatives, including the SPARCbook 3000 series, Pentium-based Tadpole P1000 series, the DEC Alpha based Tadpole ALPHAbook 1, and the PowerPC-based IBM RS/6000 Notebook N40 for which Tadpole manufactured for IBM. All of these seem to use a variation of the exact same keyboard with only the mouse buttons' shape changing between them. The assemblies can in fact be noted for having three mouse buttons, a rarity for IBM buckling sleeves keyboards. The only photos I could find of their sleeves and the birth certificate are from R. Arachelian, which show the presence of sleeves, an M6-1 birth certificate (albeit hard to make out) and a TrackPoint II-style pointing stick. IBM also confirms that the N40's stick at least is TrackPoint II. In terms of part numbers, the only solid example of one is from the Ardent Tool which lists "Lexmark P/N 1403213, Model M6" as the keyboard included on the N40. Through communication with J. Eckert who has a blog post on the SPARCbook 3000ST, I also learned Key Tronic made some of the keyboard assemblies (presumably on later examples made after April 1996) as "KTC" was observed to be written on a partial view of his keyboard's birth certificate.

J. Eckert has since posted a teardown of the 3000ST on 29th January 2022 and has shared a better photo of the keyboard's birth certificate with me. Its part number is 600515-US, and his new post clearly shows the keyboard's characteristic buckling sleeves riveting.

Lexmark Lexbook SE-10 (1994-03)

Summary: Model M6 based, produced by Lexmark, medium & small sleeve gauges

The Lexmark Lexbook SE-10 was a 486SLC/2 based machine released by March 1994. This seems to be the last Lexmark laptop of which photos are available. The keyboard assembly is very similar to the MB-1x assembly shown above but its top row more closely follows the IBM ThinkPad 500 and 510CS design these keyboards seem to be based on. In fact, the laptop itself resembles a 500 or 510CS that's been given a more 'curved' makeover. The shared DNA is more obvious than ever with this. Anyway, one interesting difference to all the other Lexmark Lexbook keyboards is that this one's keycaps are coloured grey to match the rest of the machine. In a way, this is probably the closest we're getting to an industrial grey coloured IBM buckling sleeves keyboard...

IBM ThinkPad 360 series (1994-05)

Summary: Model M6-1, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 360 series were 486SX or 486DX2 based machines first released in May 1994 and included the 360, 360CS, 360C, 360P, 360PE, 360CE and 360CSE models. As per IBM Mobile Systems Hardware Maintenance Manual Volume 2, all models seem to use the same keyboard assemblies, which were shared with the slightly later 355 series with an 84G5xxx nomenclature.

IBM ThinkPad 370C (1994-05)

Summary: Model M6-1, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 370C was a 486DX based machine released in May 1994 and shares the same 66G0xxx nomenclature keyboard assemblies as the 750 series, 755C and 755Cs as per IBM Mobile Systems Hardware Maintenance Manual Volume 2.

AST Ascentia 900N series (1994-06)

Summary: Model M6-1 based, produced by Lexmark, TrackPoint II or FSR stick, standard & small sleeve gauges

The AST Ascentia 900N was a 486DX2/DX4 based machine launched in the middle of June 1994. These machines use a Lexmark-made Model M6 based keyboard assembly, specifically M6-1 as evidenced from the barrel slider colour. The 900N was followed up by at least a 910N (which seemed to be like the 900N but with a superior battery) and 950N, both of which seemed to still use the same keyboard. However, a difference in pointing stick technology has been observed though. Early keyboards that were made when only the 900N was released seemed to use TrackPoint II pointing sticks as evidenced by my July 1994's example having a square top. However, a 950N keyboard I saw on eBay has a circular top just like Lexmark and Unicomp FSR-based pointing sticks, which is the same type Unicomp uses today on their EnduraPro keyboards. Whilst I'm not 100% sure of the performance of these sticks, this could mean earlier examples would have more performant pointing sticks as Lexmark/Unicomp FSR sticks are generally considered to be worse than any strain gauge TrackPoint revision. Anyway, these keyboards also seem partially related to the ThinkPad 500 design, and thanks to the example I own, I'm able to show the small sleeves residing under the tiny keycaps of the top row.

IBM ThinkPad 355 series (1994-06)

Summary: Model M6-1, produced by Lexmark, TrackPoint II, standard & medium sleeve gauges

The IBM ThinkPad 355 series were 486SX based machines released in June 1994 and included the 355, 355C and 355CS models. 355s use the same 84G5xxx that the 360 series used. The IBM Mobile Systems Hardware Maintenance Manual Volume 2 confirms this, although note that some revisions of this volume incorrectly refer to the 355 series as "350" in the Parts Listing section headings. But, the contents page and footer should confirm that the volume in fact refers to the 355 instead.

IBM ThinkPad 510CS (1994-06)

Summary: Model M6 based, TrackPoint II, medium & small sleeve gauges

The IBM ThinkPad 510CS was a follow up to the ThinkPad 500 using a 486 BLX2 processor and uses an identical keyboard. The only difference perceivable is the inclusion of brightness and contrast control sub legends on Q/A and W/S keys respectively. Jack @ laptop.pics confirmed the keyboards are indeed fully interchangable between 500 and 510CS. The part number for this keyboard is 73G3394.

IBM ThinkPad 755C series (1994-06)

Summary: Model M6-1, produced by Lexmark & Key Tronic, TrackPoint II (66G0xxx) or III (84G6xxx), standard & medium sleeve gauges

The IBM ThinkPad 755C series were released in several stages; 755C and 755Cs in June 1994, 755CD in October 1994, 755CE and 755CSE in November 1994, 755CX and 755CV in May 1995, and 755CDV in June 1995. Most 755C series laptops were 486 machines, however, some of the later models could come with a Pentium. Anyway, 755C and 755Cs initially shared the same keyboard assemblies as the 750 series and 370C with part number nomenclature 66G0xxx as per IBM Mobile Systems Hardware Maintenance Manual Volume 2, which only seemed to be produced by Lexmark. However, all the subsequent releases have a different part number nomenclature of 84G6xxx with Lexmark-OEM and Key Tronic-OEM examples known. The technical difference between the two groups of keyboard assemblies for this family is that 66G0xxx has TrackPoint II and 84G6xxx has TrackPoint III (deduced from the ThinkWiki TrackPoint page's information on what machines had what TrackPoint revision). Lexmark UK and Key Tronic made examples can also be distinguished from all Lexmark US ones via the difference in mouse button style - the original design had small entirely red buttons on the forward edge of the keyboard assembly, however, the updated design has larger buttons that can be slid out into a locked position to prevent the buttons from clicking. In fact, the keyboard assembly now looks entirely like the later 365 series design.

Winbook XP5 (1994)

Summary: Model M6-1 based, produced by Lexmark, FSR stick, standard & small sleeve gauges

Added 2022-10-03: The Winbook XP5 was a laptop from circa 1995 whose manual reveals has an "82-key Lexmark keyboard, 3mm travel". Earlier in 2022, I managed to snag what appears to be its keyboard assembly on eBay and can confirm it's Model M6-1 based. I don't know the exact introduction date of the XP5, but its keyboard assembly was produced as early as August 1994. It seems to be a simple reskin of the AST Ascentia 900N series keyboard assembly.

IBM ThinkPad Power Series 850 (1995-07)

Summary: Model M6-1, produced by Lexmark & possibly Key Tronic, TrackPoint III, standard & medium sleeve gauges

The IBM ThinkPad Power Series 850 was a member of a unique line of PowerPC (a RISC-based processor family) laptops originally introduced in 1994 with the ThinkPad 800. Whilst photos are not widely available, the 850 is known for using Model M6-1 keyboards, however, confirming part numbers is a bit tricky since they're (like all other Power Series laptops) not included in any of the IBM Mobile Systems Hardware Maintenance Manual volumes and their individual documents are hard to come by. Thankfully, the OS/2 Museum had a copy of the 850's hardware maintenance manual donated to them and it revealed some keyboard assembly FRUs. It turns out the 850 shares the same keyboard assemblies as the later RS/6000 Notebook 860 (itself the successor to the 850) which have a 29H81xx nomenclature. Since I do have photos of an 860 keyboard at least, I consider the matter of designation settled. One last thing to note is that the mouse buttons on these keyboards are also slide-lockable like the later 755C series keyboards mentioned above although their shape is different.

As a side note for those curious about the other Power Series laptops, one video I found showing the back of the 820's keyboard assembly by T. Mädel shows it to not be a Model M6-1 or potentially other IBM buckling sleeves keyboard.

IBM ThinkPad 365 series (1995-11)

Summary: Model M6-1, produced by Lexmark & Key Tronic, TrackPoint III, standard & medium sleeve gauges

The IBM ThinkPad 365 series were released in two stages; 365C, 365CS, 365CD and 365CSD in November 1995, and 365E, 365ED, 365X and 365XD in May 1996. As you may anticipate given Lexmark's exit from the keyboard market occurred in April 1996, the first batch has primarily Lexmark-OEM keyboard assemblies and the latter exclusively has Key Tronic-OEM assemblies. A 365E/ED/X/XD keyboard assembly was in fact used as the M6-1 representative in earlier parts of this article. Anyway, IBM Mobile Systems Hardware Maintenance Manual Volume 3 confirms variants from both stages of the 365 releases use the same keyboard FRU numbers (checked with US English FRU 41H9789), indicating keyboard assembly equivalence across the variants. The slide-locking mouse buttons are once again present.

IBM RS/6000 Notebook 860 (1996-11)

Summary: Model M6-1, produced by Key Tronic, TrackPoint III, standard & medium sleeve gauges

The IBM RS/6000 Notebook 860 was the successor to the ThinkPad Power Series 850 that dropped the ThinkPad branding in exchange for an updated GPU. The 860 seems to be the last new IBM laptop release intended to use a Model M6-1. As aforementioned for the 850, these are well known for being bearers of IBM buckling sleeves as photos of the 860's employment of these switches are widely available. In the case of the 860, these are post-Lexmark-era thus only use Key Tronic-OEM M6-1s. Whilst listings of 860 keyboard assembly part numbers seem to be unavailable, the 850's Hardware Maintenance Manual and available photos of 860 keyboard birth certificates confirm the 850 and 860 share the same keyboards with a 29H81xx FRU part number nomenclature. Something not really surprising given the machines are virtually identical apart from the GPU inside and the branding. According to IBM, parts availability for the 860 ended on 11th November 1997.

Similar attempts at buckling sleeves on laptops

Key Tronic buckling sleeves

Despite IBM's standing partnership with Lexmark and ability to produce keyboards themselves, IBM turned to Key Tronic to produce the keyboard for the IBM ThinkPad 701C series. Introduced in March 1995, the 701C (active matrix LCD model) and 701Cs (dual-scan LCD) are amongst the most well-known ThinkPads of all time due to the design of John Karidis' TrackWrite keyboard that many refer to as the "butterfly keyboard" these days. To keep things simple, the keyboard is designed to be in two halves that can slide to compact when the laptop's lid is closing. The intent was to keep the laptop itself compact but negate the need for keyboard layout compromises at the same time. The 701 series was discontinued before the end of that year due to laptops becoming generally larger, but this wasn't before the laptop became a best seller and has gone on to receive many design awards and secure a position in museums.

The keyboard made use of buckling rubber sleeves switches unique to Key Tronic, resembling rubber domes with the top of the domes chopped off. Key Tronic patented this design in August 1992 under US5298706A (Membrane computer keyboard and improved key structure) as per the sticker on the back of 701 series keyboard assemblies. Note that the patent still refers to these as "domes" despite the obvious visual difference. Anyway, the membrane is rod actuated like M3/M4 type IBM buckling sleeves, however, the keycap mounting mechanism is much more simplified.

In terms of key feel, Key Tronic's take on buckling sleeves is seen to be worse than IBM's own design. micrex22 summarised it is his review of various ThinkPad keyboards as Key Tronic's take feeling much more gummy whilst accepting the possibility this could be down to degradation due to age. From my own experience after lending a 701C a while back, I agree with this conclusion - I think they feel gummy, but I wasn't around to try these in their heyday. Anyway, whilst it's not telling of how they were when they were new, what they've become today is something we should keep in mind.

Alps KFNR buckling sleeves

ALPS Electric also had its own take on buckling rubber sleeves for use with various input devices, most notably the keyboard on the Apple PowerBook 5300 series. Released in August 1995, these were Apple's first PowerPC based PowerBook laptops and known for having hot-swappable expansion modules but also regarded as one of Apple's worst products due to 'design faults and manufacturing problems' (according to Wikipedia).

ALPS KFNR or "ALPS buckling sleeves" was fairly different to all the other variations (Key Tronic's included) in that the sleeves are topped with a white plastic stabilising and barrel slider element (which I guess is a form of key stem) that clips onto the barrel plate above the sleeves. The actual keycap-tops then clips in above both elements. The sleeve itself is noticeably smaller than IBM's implementation, and the rubber seems to be thicker as well. Another interesting fact is that IBM itself also used this switch design in Q2 1995 with the IBM KeyPad III (such as P/N 95F5446 for the Belgian/Hebrew/UK English/US English release), which is a widely available cigar box-style numeric keypad sold for use with various ThinkPads. In fact, they're sometimes sold as "IBM ThinkPad 560 Numeric Keypad Option" online. It seems these keypads were in production for the entire second half of the '90s since my example is from 1999.

Once again, I do not believe these are as good as IBM's native implementations of buckling sleeves. Your mileage may of course vary, but I find ALPS buckling sleeves to be less tactile, feel noticeably worse if you press a key off-centre, and the keycaps themselves are prone to wobbling to a severe degree. The small sleeves (in comparison to IBM's implementations, shown below) and seemingly flimsy stabiliser/slider elements are what cause the issues.

Their impact & legacy

Unlike in the previous article regarding the M4s, it's much easier to gauge the opinion regarding devices that used M3s or M6s from back in the day - and for the most part, it's good news. Whilst the sporting machines themselves could be criticised, the keyboard was usually not subject to criticism. Starting with the OG itself, the L40SX, the keyboard was considered one of the highlights of the machine. A week after its debut, Computerworld in its 1st April 1991 noted "analysts and users have praised the full-size sloped keyboard", quoting "It's got the best keyboard of any [laptop] by far". In InfoWorld's 8th July 1991 issue, their review of the IBM PS/2 L40SX concluded the keyboard as "Very Good", which was their second-highest rating possible. Rated in the same league as its "list of features and options", PC Magazine on 14th May 1991 said the keyboard is one of the laptop's primary strengths. Moving on to the very similar keyboards used by Lexmark for their OEM laptops, PC Magazine's August 1993 issue noted in their review that the Hyundai Courier Spectra's keyboard is the highlight of the machine saying "On a positive note, the keyboard gives good resistance and has full-size keys that are well laid out".

ThinkPads have always been widely praised for their keyboards, with classic ThinkPads sporting Model M6 or M6-1 being no different. The ThinkPad 700C review in the 14th December 1992 issue of InfoWorld gave the keyboard it's second-highest rating "Very Good", noting "We were very pleased with the ThinkPad's keyboard. The keystroke action was quick and precise and had a solid feel to it..." The same PC Magazine issue from August 1993 mentioned before also noted the IBM ThinkPad 720C's keyboard as "...exceptional. The spring-loaded keys give satisfying resistance when typing." For a later buckling sleeves user, the 9th January 1995 issue of Computerworld said the ThinkPad 755CD's keyboard is "as good as notebook keyboards get".

In contemporary times, laptops that have IBM buckling sleeves keyboards still generally receive high praise. In EEVblog's teardown of an L40SX in July 2014, he said "the thing I absolutely loved most about this machine was the keyboard". Nostalgia Nerd said in his May 2021 video on the IBM PS/2 L40SX that its keyboard is "by a country mile, the nicest laptop keyboard I have ever had the pleasure of typing on". In Jakob's extensive list and review of DOS-based palmtop PCs, the Lexmark Lexbook MB-10 and MB-15 were both noted for their excellent keyboards. Modern text or videos describing ThinkPad buckling sleeves keyboards specifically are not very common, but generally, early ThinkPad keyboards altogether are clumped together in the 'good category'. That said, on deskthority, ibmfiles site owner micrex22 concluded that the IBM buckling sleeves keyboard assemblies of the IBM ThinkPad Power Series 850 and ThinkPad 360PE were the best in their comparison of many other ThinkPad keyboards.

Summary

Thank you for sticking by yet another mammoth article from Admiral Shark's Keyboards! This one turned out to be far longer than I had anticipated, even in terms of research hours let alone words. It turns out IBM's buckling sleeves keyboards on laptops was more widespread than a lot of people (including me) thought - I hadn't thought about the possibility of non-IBM laptops using these keyboards when I started my research back in August 2021. The never-ending discoveries coupled with the fact photos of the insides of many of these laptops are so few made research long and sometimes frustrating. But, I think it's been all worth it as I think this should clear up questions on the topic quite a bit. I think we can conclude that buckling sleeves literally dominated IBM laptops in the early to mid-1990s, but we've also found that through Lexmark's OEM activities other companies had access to this keyboard technology as well. I think I was most surprised by seeing a Hyundai laptop with an IBM buckling sleeves keyboard assembly, so I guess if you own a Hyundai Tiburon/Coupé, you can have a matching Model M6-like bearing laptop! ;)

Anyway, one of the great things this time around is that I can present positive public opinion on these keyboards rather than making you rely on my word alone. Whilst M4s certainly have their fans, the fact they're not a big name like ThinkPad means the opinions on them are usually obscured unless you're on r/ModelM Discord or something. Obviously, my own opinion of these keyboards is high given I'm interested enough to explore the topic. But clearly, IBM knew what they were doing with their laptop keyboards and thus ThinkPads have held a reputation for good keyboards for all this time. As I said in the summary of the Model M4 article, I still know these will not satisfy everyone but I still recommend everyone try them at least once - you may be pleasantly surprised. Again borrowing from my words in the former article, we know from our current perspective that demand for cheaper and thinner machines (and thus cheaper and thinner keyboards) lead to buckling sleeves falling into disuse by the late 1990s, with only Unicomp continuing to manufacture Model M4s until 2010. Hopefully, this article will let people know that these in fact existed and made an impact back in the day and are still well regarded now! Cheers, and once again, big thanks to everyone that contributed!

To end, here are some infographics you can look at a share if you like!

Further reading & resources

Site resources

External resources

Videos

Magazines

Part/FRU number lists

My buckling sleeves keyboards

Acknowledgements

Just like the predecessor article, this one was a huge undertaking that required a lot of things to come together to produce this well-illustrated article. I wish to fairly acknowledge everyone who has contributed to this article via donation or request and highlight those who I've used as a source of photos. Some photos have been sourced from volatile places such as eBay listings as a last resort option, in which case, such photos are included in a way believed to be compliant with UK's fair dealing law - they are used not for profit, but research, commentary and analysis of this subject.

If there are any issues with citations or use of images, please let me know (details on the Contact Me page).

Corrections & changes