Revealed: The Story of the Model M4 family

  • Published 03 July 2021
  • Updated 13 October 2021
  • Est. Reading Time: 39.97 minutes

When you hear the name "Model M", you immediately think of a beige, clicky titan of a keyboard. And no one would fault you for that notion given the buckling springs keyboards' fame and glory. However, the majesty of the Model M family expands far beyond that. Many people don't realise this due to the fact resources and data on these outliers in the family is relatively scarce. But I want to change that, starting with the Model M4 and M4-1 keyboards. These are perhaps my favourite non-typical Model M variant and they've made somewhat of a renaissance amongst the folks on r/ModelM. And so, this is the Model M4 story and the definitive article regarding these keyboards. We'll be taking a look at its origins as a laptop keyboard assembly, what makes these keyboards unique, and the Model M6 ThinkPad keyboards that they inspired.


Introducing the Model M4 and M4-1

Officially called the IBM Space Saver Keyboard and not to be confused with the IBM Space Saving Keyboard or SSK, the Model M4 and M4-1 were two keyboards produced from 1993 to around the late 2000s. Awareness of them and their availability are what I describe as 'middling'; most IBM keyboard enthusiasts at least seem to know they exist and their prices are not absurd (yet). But, they don't have much in the way of a reputation, to be honest. However, I believe they are underrated for what they are and have an interesting history and legacy that's worth discussing.

So, what are they? Succinct to a single sentence, they're buckling rubber sleeves keyboard with a tenkeyless layout that's been slightly compressed. In fact, here's a photo of an M4 and M4-1:

Model M4 (left) and M4-1 (right)
Model M4 (left) and M4-1 (right)

As shown, the difference between the M4 and M4-1 is the pointing stick and its left and right mouse buttons. The M4-1 uses a TrackPoint II strain gauge-based pointing stick that's also found in countless early ThinkPad laptops and the later Model M13 full-size buckling springs keyboard. Being an early implementation, it's not as performant as the TrackPoint IV sticks found on the latest Lenovo ThinkPads but it's still certainly useful and can be improved through software assistance.

The Model M4 design is both different and familiar to other Model Ms. Obviously, the switches (and thus the internal assembly) are different. But, IBM tried to keep the design of the M4 somewhat familiar. The case features the same classic wedge shape and oval-shaped badge that many Enhanced Keyboards, Space Saving Keyboards and their derivatives had.

They also feature robust SDL modular connectivity that can use the same cables as other AT or PS/2 SDL-sporting Model Ms used. The M4-1 specifically used the same Y-split dual PS/2 cable the Model M5-1 and M5-2 trackball keyboards used for keyboard and mouse connectivity. A notable improvement (or visual disruptor, depending on your perspective) over the SSK is the inclusion of lock-light LEDs. The M4s never partook in the 'generations' IBM and later Lexmark maintained for the buckling springs keyboards. Although, branding had some nuances that will be explained in a later section.

M4s would have been used in a variety of roles, including server monitor, sysadmin, and docked laptop setups as evidenced by the fact many M4 part/FRU numbers are typically included in mid to late '90s IBM ThinkPad hardware maintenance manuals. By 1999, IBM succeeded the M4 with the rubber dome based IBM Space Saver II Keyboard (model RT3200), but Unicomp continued to produce the keyboard as the Unicomp Mighty Mouse (codenamed "Surf") until the end of the 2000s.

The origins - IBM PS/2 L40SX, CL57SX and Model M3

So, in the beginning, I mentioned a story to tell. As it turns out, that story is interesting from the beginning - in 1991. IBM in some ways was still a fairly 'vertical' company during the late '80s; even after the introduction of the IBM PC in 1981 that used many off-the-shelf components, they especially still controlled much of the production process for their keyboards. This led to them naturally reusing whatever they could, likely for the economies of scale benefit and making the research cost of the products more digestible. With this in mind, it shouldn't be surprising that the Model M4 family of keyboards originated as a keyboard assembly for a few early pre-ThinkPad laptops and a numeric keypad attachment for one of them. So, what are these M4 predecessors then?

First up is the IBM Personal System/2 L40SX laptop. The L40SX was one of the first portable PCs from IBM that resembled a laptop as we know them. Launched in March 1991, it was 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. Of particular note is the keyboard, and you can see right away how familiar it is - it's the same layout and keycap profile as the Model M4! Indeed, the L40SX keyboard assembly was the first to feature IBM buckling rubber sleeves switches that are central to the M4 design and quality key feel.

IBM PS/2 L40SX keyboard deck (credit: Uberman765)
IBM PS/2 L40SX keyboard deck (credit: Uberman765)

By many accounts, the L40SX has achieved a sort of legendary status due to how much key travel and key feel IBM extracted from such a low-profile design. This also extends to the complementary numeric keypad IBM made for the L40SX - the Model M3 (eg. P/N 1396199 for the US market version). This is where these keyboard designs first officially tie into the Model M family.

Model M3 rear stickers - US-made (credit: Dario @ Dario'sCope) and UK-made
Model M3 rear stickers - US-made (credit: Dario @ Dario'sCope) and UK-made

The numeric keypad sports the same buckling rubber sleeves switches that the host L40SX uses, a pass-through port for a PS/2 mouse, and connects to the host with a modular 8P8C (aka, "RJ-45") to keyboard/mouse signal-combined PS/2 cable. In case you're wondering, the L40SX keyboard assembly itself does not bear a Model M designation despite the clear relation. However, the assembly does still sport Model M-like stickers and a 'birth certificate' though.

IBM PS/2 L40SX keyboard assembly backplate (credit: Uberman765)
IBM PS/2 L40SX keyboard assembly backplate (credit: Uberman765)

The pre-Model M4 story doesn't end there. Following up from the L40SX about a year after its release was the IBM PS/2 CL57SX from March 1992. The CL57SX was known for being the first IBM laptop with a colour TFT display, having an integrated trackball pointing device, and is an immediate ThinkPad predecessor. As you can see, the keyboard assembly is very familiar once again. There are some visual differences beyond the fact it's black too. Most notably; what would be grey keycaps on a standard white/grey IBM Enhanced Keyboard colour scheme have gold lettering, the caps lock keycap is stepped unlike the L40SX before it and the Model M4s after it, and there is a numeric keypad overlay feature on the keyboard much like the IBM Space Saving Keyboard.

IBM PS/2 CL57SX keyboard deck (credit: Jack @ laptop.pics)
IBM PS/2 CL57SX keyboard deck (credit: Jack @ laptop.pics)

Speaking of the CL57SX, it's also worth noting that it had a Japanese-only analogue - the IBM PS/55 N27sx. The appearance of the computer and the keyboard assembly are for the most part the same, however, the N27sx uniquely has a Japanese JIS version of the keyboard assembly that was not available on the L40SX or Model M4s. The JIS keyboard also has some unit sizes modified - the ISO-style enter is now full-size but at the expense of culling about a quarter of the unit size of the navigation keys to the right of it. I'll let you decide if that's a worthwhile compromise or not.

Whilst it's certainly possible other examples of similar buckling sleeves keyboards assembly exist before the Model M4s, I believe these are the main and most well-known examples.

The architecture underneath the skin

So, that backstory is all well and good but what makes these keyboards special, then? As mentioned a few times, these use switches called buckling rubber sleeves. These are not the same as rubber domes for two reasons; they're neither shaped like a dome and the rubber component itself plays no part in bridging the sensor (the membrane) underneath. Rubber domes typically pull triple duty in their operation:

  1. They play a direct role in actuation; for membranes, they bridge contacts together, but for PCB-based versions, they can either have a piece of conductive or capacitive material on the bottom of the dome to use with 'interacting' with the PCB.
  2. They provide tactile feedback to help the user feel the keys and confirm they've made a press.
  3. They push the keycap but up to the top after being depressed.

IBM's implementation of sleeves only does the latter two since a rod on the keycap is used for bridging contacts on the membrane inside the keyboard's assembly instead. Emphasis on IBM though, since other non-IBM or non-IBM-affiliate manufacturers may have their own versions of sleeves switches that differ greatly. On the topic of the keycaps, stabilisation of it is achieved with the two hooks on either side of the keycap that clip into the plate the sleeves sit on.

Model M4/M4-1's buckling sleeve and the keycap mechanism
Model M4/M4-1's buckling sleeve and the keycap mechanism

Membrane-based rubber domes switches have the reputation of feeling mushy, especially when bottoming out in key travel due to the dome depressing against the membrane. On IBM sleeves, the impact at the bottom is much for solid due to the sleeves inherent design and the solid 'interface' the rod on the keycaps provide.

Despite the fact sleeves sound advantageous on paper and have a good overall reputation in IBM circles, they're a relatively uncommon type of switch these days due to one main factor; the need for increasingly lower-profile switches and the fact sleeves would scale poorly when scaled down. This was true of rubber domes too, but this is what necessitated the scissor-switch design where each rubber dome switch got its own stabilising support structure. On sleeves switches, the rubber component expands outwards across a fair amount of underside keycap surface area and thus becomes its own stabilising element. So, scaling down a sleeves switch would negate this inherent feature. As such, the days of sleeves in laptop keyboards are long over.

Moving on into the internal assembly structure, we get a mix of familiarity and improvement to the typical Model M assembly. As you're familiar with, the M4s are built with three layers. On top, we have a barrel plate (or simply top frame in Unicomp's dictionary) that is used for guiding switch and keycap positions. As the name implies, it's a field of barrels on a unifying plate that sits at the top of the assembly. The key difference in this implementation over the buckling springs' one is that the key component of the switch (the sleeves) sits on top of the barrel instead of underneath (like the spring and pivot hammer sets in buckling spring designs).

In the middle of this 'keyboard sandwich' is the membrane. As you may suspect, the membrane's job is to register keypresses when the rods on the keycaps press a given two contacts on said membrane together. Whilst the M4 membrane is not shared with any buckling springs Model Ms, the membrane interfaces with its controller board in the same way - flex cables socketed with triomate-type connectors.

Model M4/M4-1's membrane flex cables and controller interface
Model M4/M4-1's membrane flex cables and controller interface

The final layer is a metal backplate on the back. The backplate on the M4s is the thinnest I've ever seen on a Model M design, but it has one fundamental advantage over Enhanced-style buckling springs backplates - it doesn't need to be held on with rivets! Instead, it uses a sort of teeth retainer design that seems to be much more robust since I've yet to encounter an L40SX or Model M4 with a broken tooth. There is an exception to this that will be explained in the following section about the numeric keypad options. But sufficed to say, the keyboards of the Model M3/M4 family feature robust internal riveting that has lasted 20+ years without incident.

Model M4/M4-1 keyboard assembly backplate
Model M4/M4-1 keyboard assembly backplate

Now, let's look at the TrackPoint on Model M4-1s. As said near the beginning, M4-1s use the TrackPoint II strain gauge pointing stick designed by IBM. Strain gauges are sensors that can measure the strain being applied to an object. For this TrackPoint implementation, these sensors are mounted on the bottom of a pointing stick emanating from in between the G, H and B keys. The measured strain values are used to calculate which way and how fast the cursor should move. For more in-depth information, you can see patent US5521596A. The current IBM/Lenovo TrackPoint IV pointing sticks continue to operate with this technology, but as mentioned near the beginning, it is more performant. Starting with 1994's TrackPoint III, IBM introduced a negative inertia transfer function into TrackPoint's firmware that counteracts the general feeling of sluggishness (ie, having inertia) via acceleration (explained more in patent US5570111A) which the M4-1 was never upgraded to take advantage of.

Model M4-1 TrackPoint II pointing stick
Model M4-1 TrackPoint II pointing stick

The TrackPoint pointing stick is complimented with two buttons at the bottom lip of the keyboard for left and right mouse clicks. These are blister-style buttons and are in my opinion the weakest point of the keyboard's design. They will do the job and are not as mushy as the mouse buttons found on the later Model M13 keyboards, but microswitch-style buttons would be much preferred. Anyway, the buttons extend from a membrane flex cable and are kept in place by two retainer clips on the lip of the keyboard. The button caps are then placed on top of these are held in place on four slots by pressure from the keyboard assembly once the keyboard is screwed together. It's a fairly simple solution and was most likely an improvisation, since as we know, this keyboard is a repurposed laptop keyboard assembly that never had a TrackPoint (thus the need for mouse buttons) to begin with.

Model M4-1 left and right mouse button mechanisms
Model M4-1 left and right mouse button mechanisms

The numeric keypad option

Accompanying the M4 and M4-1 keyboards are the Model M4-1 numeric keypads. These spiritual successors to the true Model M3s are 18 or 19-key sidebar attachments that connect directly to the Model M4 or M4-1 via a modular 10P10C to modular 10P10C (aka, "RJ-50" to "RJ-50") cable.

Model M4-1 numeric keypad
Model M4-1 numeric keypad

I've already discussed at length the inner workings of the M4-1 numeric keypad in my earlier teardown article about my 1995 Lexmark example. As a quick summary of what was discovered and shown, the M4-1 numeric keypad is a 'brainless' device in that it possesses no controller of its own. Instead, the connection between the keypad and the host keyboard directly connects the keypad's membrane to the host, meaning it is the keyboard that acts as the keypad's controller This means you can't normally use them as separate numeric keypads for any old system unless you build your own controller for it. Another unique feature about their construction is that they don't use the same 'teeth' rivets that the M4/M4-1 keyboards use. In fact, these use melted plastic rivets that most Model Ms use.

Model M4-1 numeric keypad's assembly
Model M4-1 numeric keypad's assembly

Despite this revelation, the assembly still seems well secured as the keypads I've observed seem to all have their rivets intact. And if they somehow pop, I can imagine an M4-1 numeric keypad version of the bolt mod would be a piece of cake anyway.

Lexmark, Key Tronic & Unicomp - a trio of OEMs

As characteristic for many Model Ms, the M4 family was not produced by a single company throughout its lifecycle. The L40SX keyboard assembly they were based on were largely produced by IBM itself at the IBM US factory in Lexington, Kentucky and IBM UK factory in Greenock, Scotland. However, M4 production proper seems to have started at Lexmark, since by 1993, the divesture of IBM Information Products Corporation was already two years prior. Despite Lexmark using IBM UK/Greenock production space for other keyboards in some capacity, M4s were never produced in the UK. Anyway, in addition to producing IBM-branded M4s, Lexmark themselves also marketed the keyboard as its own product.

Lexmark-branded Model M4 keyboard and Model M4-1 numeric keypad topside (credit: arkanoid)
Lexmark-branded Model M4 keyboard and Model M4-1 numeric keypad topside (credit: arkanoid)

After Lexmark dropped keyboard production in April 1996, Key Tronic provided the immediate continuation of M4 production. Key Tronic produced M4s can be distinguished from the back even if the labels and 'birth certificate' are missing since they sport a dip switch bank (or a sticker covering them) used for altering the scancodes sent out by the numeric keypad attachment. Otherwise, they seem to be functionally identical to Lexmark-produced keyboards. Of note however is that as of the time of writing, I've not encountered any black Key Tronic-made M4s.

Key Tronic Model M4-1's dip switch bank (credit: ripster)
Key Tronic Model M4-1's dip switch bank (credit: ripster)

One thing vintage ThinkPad enthusiasts might be curious to know is that Key Tronic seemed to also use the same buckling sleeves as Lexmark. Key Tronic's buckling sleeves implementation used on a few mid-'90s ThinkPad keyboards have a less than stellar reputation these days due to how they seemingly turned gummy over time (for example, the sleeves used on the IBM ThinkPad 701C/701CS TrackWrite keyboard assembly). That is not the case with these, thankfully.

The numeric keypad saw a minor revision under Key Tronic too. Or more likely, they introduced options for different physical layouts (which would explain why the keyboard now has switches to alter numeric keypad's scancodes). As shown below with this Key Tronic-made numeric keypad, an extra key has been gained by splitting the 2-unit plus key into a 1-unit plus and enter key. What used to be the 2-unit enter key is now a tab key.

Key Tronic Model M4-1 numeric keypad P/N 84H8537 (credit: Brandon @ ClickyKeyboards)
Key Tronic Model M4-1 numeric keypad P/N 84H8537 (credit: Brandon @ ClickyKeyboards)

By at least 1999, Unicomp seemed to have replaced Key Tronic as the OEM producing Model M4s. Most M4s produced by Unicomp in 1999 were IBM branded and explicitly stated they were made for IBM on the back. The black M4-1 pictured throughout this article is an example of this. However, the story surrounding the introduction of the Mighty Mouse (their marketing name for the keyboard) is a bit muddied. When u/funkmon conducted a Unicomp Q&A for r/ModelM, Unicomp claimed that the Mighty Mouse was recreated in the image of the M4 for a client that subsequently backed out of the arrangement at the last minute and thus was forced to sell off the keyboards they had produced. This seemingly conflicts with the notation that Unicomp simply continued production for IBM and later sold keyboards of the design to other parties. M4-1s were most definitely produced for IBM in the final years of the '90s, since as mentioned right at the start, most ThinkPad hardware maintenance manuals of the period list part numbers of M4-1s available for purchase, and all examples of these late part numbers seem to be Unicomp-produced keyboards. A possible resolution is that Unicomp did a round of producing M4s for IBM, something happened to the tooling (maybe they were discarded), and then the unnamed client subsequently asked for the keyboard to be recreated sometime in the 2000s. This should have happened before or around 2004 since that's the earliest year for Unicomp advertising of the keyboard I can find.

Crop of Unicomp's 2004 Mighty Mouse advertisement ([see the full version here
Crop of Unicomp's 2004 Mighty Mouse advertisement ([see the full version here

Regardless, I can confirm Unicomp did not significantly alter the design of the keyboard besides adding their typical lock symbols to the lock-light overlay and later discontinuing green Alt and SysRq key legends. In fact, one oddity of the Mighty Mouse is that Unicomp continued to produce them with strain gauge-based TrackPoint II pointing sticks instead of using their force-sensitive resistor based pointing sticks. You can see the "Pointing devices comparison" on my TrackPoint, pointing stick, and UltraNav keyboards page to learn more about the differences between TrackPoint II and Unicomp's pointing stick. But sufficed to say, TrackPoint II is widely considered to be the better technology of the two. Whilst I cannot confirm the exact date, it seems Unicomp finally retired the TrackPoint-equipped Mighty Mouse around 2008 and all other variants around 2010.

To end this section, here's a general guide to the variations in IBM oval badge design used across M4s and M4-1s over the years. Self-branded Unicomp Mighty Mouse keyboards had a blanking insert in place of the IBM oval badge.

Model M4/M4-1 badges - clockwise from top-left: Lexmark M4, Key Tronic M4-1 (Brandon @ ClickyKeyboards), Unicomp M4-1, Lexmark M4-1
Model M4/M4-1 badges - clockwise from top-left: Lexmark M4, Key Tronic M4-1 (Brandon @ ClickyKeyboards), Unicomp M4-1, Lexmark M4-1
  • Lexmark-made beige M4s used flat blue IBM text on grey background
  • Lexmark-made beige M4-1s used raised grey IBM text on grey background
  • Lexmark-made black M4-1s used raised blue IBM text on black background
  • Key Tronic-made beige M4-1s used raised silver IBM text on grey background (however, this could wear off into simple grey text)
  • Unicomp-made beige M4-1s used raised silver IBM text on grey background
  • Unicomp-made black M4-1s used flat blue IBM text on black background

FAQ, potential issues and resolutions

How big are the M4 and M4-1?

The keyboard's dimensions are 33cm (l) x 6cm (d) x 3.8cm (h) or 12.9in x 2.36in x 1.46in. The numeric keypad's dimensions are 9.2cm x 6cm x 3.8cm or 3.6in x 2.36in x 1.46in. The approximate accuracy is about +/- 0.2mm.

What materials were the M4s made out of and how are the letters printed?

Whilst I can't guarantee this is the case for all M4s throughout their production lifetime, I've seen many Lexmark-era M4 and M4-1s pressed with "PC + ABS" stamped in the plastic, indicating the case is made with a polycarbonate/acrylonitrile butadiene styrene blend of some sort. Beige M4 keycaps look like they have dye-sublimated letters and many have retained their keycap texture over the years, indicating the keycaps are likely made from PBT (polybutylene terephthalate) as alternatives like ABS wear down and start 'shining' much more often and quicker. The black ones use pad-printed letters instead (much like black Model M13s), but the keycaps on my 1995 Lexmark and 1999 Unicomp M4-1 examples once again sport robust texture that can indicate they too are made from PBT.

How much should I pay for one?

At the time of publication, I would consider the $50 USD for the keyboard and $30 for the numeric keypad to be a good deal in the United States for used examples. Prices for anything vintage IBM is higher in the UK and Europe, thus I would say £60/€70 and £35/€40 or the best acceptable prices for keyboard and numeric keypad respectively you may see.

How much did these originally cost when they were in production?

I've been unable to find the price for original IBM-branded examples. However, in 2004, Unicomp's Mighty Mouse cost $99 for "pearl with mouse", $99 for "raven black without mouse", and $109 for "raven black with mouse". Adjusted for inflation, those prices would now be $141 and $155 respectively at the time of this article's publication.

Out of the three OEMs, is there one that is definitively better than the others?

This is really hard to say since it's not as clear-cut as comparing different generations and OEMs of Enhanced Keyboard. I believe all three OEMs produced similar quality keyboards in the '90s, so the decision would be a matter of buying the newest and visually clean example you can find due to how 'sensitive' the sleeves are to dirt and dirt. I specifically said in the '90s since apparently later Unicomp Mighty Mouse keyboards had slightly wobblier keycaps, and as documented above, they changed some of the keycap legends to a less cool variety. However, mechanically speaking, any M4 or M4-1 in good condition should be a good purchase.

Can I still get parts for these from Unicomp?

I've directly asked Unicomp whether they still have spare components or even any capacity to produce Mighty Mouse components. They do not have the capacity to produce new components nor do they believe they have any spare components left over.

Alternatively, can the L40SX keyboard assemblies and Model M3 numeric keypads be used as their own input devices on modern PCs?

Both can technically be converted. The L40SX keyboard assembly connects to its host via membrane flex cables that can be socketed with Triomate family 2.54mm connectors. As such, they could possibly be wired to a custom converter in a similar fashion as many IBM Wheelwriter keyboard conversion mods. I have not attempted or written about the procedure as my intention is not to promote mods that could result in working systems being discarded. However, I am much happier to document non-intrusive conversion techniques for the Model M3 itself since it's rather simple, doesn't destroy a system in the process and can give many of these numeric keypads a new lease on life. I have discovered that its unified PS/2-like connector is wired to the following pinout (socket-side, female view). With this in mind, it should be straightforward to wire a converter for the keypad with a Teensy 2.0 or Pro Micro microcontroller. If you want to make use of both the keypad and the mouse passthrough port, QMK can be used to convert both signals. However, if you only intend to use the keypad, QMK or Soarer's Converter are viable options.

How do I remove their keycaps?

All IBM buckling sleeves keycaps can be safely removed with a wire keycap puller. Simply, latch the two wire loops underneath the keycap and pull up. I'd recommend keeping the wire loops as close to the keycap's edges as possible to minimise having to touch the sleeves themselves.

The key feel seems a bit scratchy. Is that normal?

Unlike buckling springs where the mechanisms are protected in the barrels by the keycaps, the buckling sleeves and keycap clips are exposed in the 'open'. This naturally leads to dirt, dust and liquid being quite the problem for these keyboards over time, especially if the keyboards are not kept well by one or more of their previous owners. Thus, a scratchy feeling could be due to either component being unclean. I would first recommend using compressed air at a moderate distance (say, half a metre) to clean under keycaps whilst they are still attached (to prevent the sleeves from being blown away), then remove the keycaps and give them a soak. Try to avoid any hard chemicals in your solution, especially for black M4 and M4-1 keycaps since their text is technically perishable. Soap in lukewarm water should be enough. Whilst the keycaps are off, you can also try using a throwaway toothbrush to remove any dirt stuck to the barrel plate. From my experience, these actions should be enough to mitigate most scratchiness that could be present. However, if you believe it is still scratchy, you can try applying some lubrication to the keycap clips as a last resort.

The spacebar is too rattly and loud. Can I lubricate it?

Yes! Like most Model M spacebars, the stabiliser is a wire-based implementation that you can easily lubricate (and clean up if you need to reverse the procedure). I have had success with applying white lithium grease to suppress spacebar rattle noise.

The TrackPoint seems very sluggish. Can I improve its performance?

Even on the latest 2008 Unicomp Mighty Mouse, the pointing stick implementation used is IBM TrackPoint II from the early 1990s. As such, the technology works best on the low-resolution displays of the time. However, there are ways to improve the performance to what I consider acceptable levels. The most obvious thing to try is setting your operating system's mouse acceleration to its maximum possible level. For Windows users, KovaaK's open-source Interception Acceleration (InterAccel) driver is an extremely handy tool for boosting a mouse's performance beyond standard OS and driver levels. Just be sure to disable the driver before playing any online PC games as some anti-cheat software are known to be triggered by InterAccel. Unfortunately, there is no present analogue for Linux or macOS that I'm aware of, but I am in the process of developing a cross-platform alternative that I hope to release sometime in late 2021. Stay tuned on the r/ModelM subreddit for news about it.

IBM 5535-ZAD - a distant cousin?

So, we've come to the end of the Model M4/M4-1 saga proper but there's surprisingly still more to the story. The M4 family started off as a spawn of a laptop keyboard assembly, but it, in turn, spawned off several more. As such, let's start with the most mysterious one - the IBM 5535-ZAD. This awesome leviathan is a rather sizeable machine, looking like the ThinkPad A31p, G41 or W700 of its day. It was a 486-era machine running at 50MHz and had a 12.1" TFT VGA LCD display, however, even the base specs are presently unknown. The example aichi JAPAN (the singular source of information on the machine) sports 36MB RAM in a 4 + 16 + 16 configuration and a 520MB hard disk drive. But, the specs are not what I'm presently interested in - it's the keyboard it has. You might have already noticed that the keyboard's keycaps have a certain familiarity to them; they're using M3/M4/M4-1 profile!

IBM 5535-ZAD's keyboard deck (credit: aichi JAPAN)
IBM 5535-ZAD's keyboard deck (credit: aichi JAPAN)

By the looks of it, it's an evolution over the IBM PS/55 N27sx's keyboard design. The layout is now near-standard tenkeyless Japanese JIS, with modifier keys now standard Enhanced Keyboard-like 1.5-unit sized and the navigation cluster is returned to a 2x3 block. It also has an IBM Space Saving Keyboard-style numeric keypad overlay on the alpha and symbols keys too. Thus, I would summarise that this would be a nice keyboard to use. Unfortunately, the only source of information on this machine could not elaborate whether the mechanism is the exact same buckling sleeves we know, but the keycaps say it is most likely is. I'm always on the lookout for new bits of information regarding this machine and its keyboard. If you happen to anything about them, please get in touch.

Model M6 & the golden age of ThinkPad keyboards

As the final piece of our M4 story, we must cover what came after - the Model M6 ThinkPad keyboard assembly family. We've already looked at one unique laptop keyboard assembly based on the M4, but these M6s were a bit more of a radical development in terms of form and layout. One of the hallmarks of the ThinkPad keyboard is the fact they no longer compromised on the alphanumeric keys and the functions around them. Instead, IBM moved most navigation keys to the top-right corner and moved the arrows further down. The new escape, function, navigation and arrow keys were now smaller than they were on the M4s, but, the modifier, enter and backspace keys were at least no longer as compromised.

ThinkPads have long been heralded for having a strong typing instrument attached to them. These M6s are essentially what started this reputation, ensuring what was good about the L40SX and CL57SX that came before them continued. So, let's take a look at what changed (or stayed the same) during the M3/M4 to M6 transition.

IBM ThinkPad 365X/365XD Model M6-1 keyboard assembly (note that this is a Japanese keyboard with Thai language stickers applied by a previous owner)
IBM ThinkPad 365X/365XD Model M6-1 keyboard assembly (note that this is a Japanese keyboard with Thai language stickers applied by a previous owner)

Let's start by saying not all '90s ThinkPad keyboards are necessarily Model M6s. As you may expect, most that count have a 'birth certificate' on the back of the assembly that proudly identifies itself as an "IBM Model M6". M6s were typically produced by Lexmark until April 1996, after which Key Tronic picked up production in the same manner they did for the M4s. When IBM moved from sleeves to small rubber pegs and scissor switches at the end of the '90s, the M6 designation was no longer found on the back. Although, its legacy persisted and the layout the M6 introduced was in use with minor revisions until 2012 when Lenovo introduced the AccuType island-style keyboard as its replacement across the entire ThinkPad family. Today, ThinkPad folk simply refer to any 1992 to 2012 ThinkPad keyboard as the 'classic ThinkPad' keyboard.

IBM ThinkPad 365X/365XD Model M6-1's birth certificate
IBM ThinkPad 365X/365XD Model M6-1's birth certificate

M6s produced by Lexmark and some later ones by Key Tronic feature the same sleeves on their alphanumeric keys that the M4s used. In fact, I tried swapping some of the sleeves between this 365X/365XD keyboard assembly and one of my M4-1s and found it didn't alter the key feel significantly. As shown in the photo below, the smaller keys get their own smaller sleeves to match, but they don't feel different to the alphanumeric keys. That said, slight differences in key feel can be observed between different M6-equipped ThinkPads, but such variance could possibly be attributed to the condition of the sleeves. M6s in general feel slightly different to M4s despite the same sleeves, likely due to the thing that has changed significantly - the keycap mounting mechanism. Gone are the two clips on either side of the keycaps, now replaced with a central mount where the actuation rod previously stood. This new mount attaches to an actuation slider stuck in the barrel itself. I guess you could call this a 'sleeve with slider' design. Anyway, however different, the key feel is not a revolutionary difference and I'm unable to pick a favourite between them.

IBM ThinkPad 365X/365XD Model M6-1's switches
IBM ThinkPad 365X/365XD Model M6-1's switches
IBM ThinkPad 365X/365XD Model M6-1's key sliders
IBM ThinkPad 365X/365XD Model M6-1's key sliders

The naming convention of the M6 is similar to that of the M4; M6-1s have TrackPoints, M6s don't. However, given TrackPoints were added to the ThinkPad design almost immediately (relatively speaking), "95%" of possible sleeves-featuring ThinkPad keyboards will be M6-1s. They can feature TrackPoint II or III pointing sticks - M6-1s were either near or completely out of production by the time TrackPoint IV was released. This means some M6-1s like this 365X/365XD keyboard assembly are graced with a TrackPoint that has the negative inertia function built-in, providing a significant performance improvement over the TrackPoint II sticks all M4-1s are stuck with. Besides referring to a given ThinkPad's hardware maintenance manual, a possible way to quickly tell a TrackPoint II or III stick apart is the tip and rotation of the stick itself. If you refer to the photos of the M4-1's TrackPoint shown previously, you can see that they take the form of a non-rotated stick with what looks like a debossed square on the tip. As shown below, the TrackPoint III stick seems to be slightly rotated and has a flat/flush tip. However, both sticks are still compatible with each other's caps.

That said, I haven't confirmed this for every single M6-1/ThinkPad. Exceptions could exist.

IBM ThinkPad 365X/365XD Model M6-1's TrackPoint III pointing stick
IBM ThinkPad 365X/365XD Model M6-1's TrackPoint III pointing stick

So, you may have noticed I've only been using a 365X/365XD keyboard assembly as a demonstration of the Model M6. If you're wondering if it's representative of all Model M6s, the answer is more or less. However, thanks to the help of Jack @ laptop.pics, find below a brief gallery of other ThinkPad buckling sleeves keyboards and Model M6-1 birth certificates confirming various ThinkPad's use of Model M-family keyboards.


And so, we're finally at the end of this story. Thank you for sticking by and reading the largest article written for Admiral Shark's Keyboards to date! I'm quite the sucker for less-known things and nuances, so I hope this article has filled the significant void of a source of information of the Model M4 and M4-1 and IBM buckling sleeves keyboards at large! To summarise then, we've learnt that the Model M4 family was a bridge between IBM's earlier PS/2-family and their later ThinkPad series portable computers, also providing a highly tactile but slim and quiet alternative to buckling springs for desktop users. These probably seem like mundane rubber dome keyboards to most are actually much more than that, in fact not sporting rubber domes as we know them at all and having quite a bit of classic Model M DNA inside when you peel away the initial preconceptions (and the screws).

Will these satisfy everyone? Probably not. They're still not an exact buckling springs-like experience by any means, some may just not like the inherent 'roundness' feeling most rubber-based switches have, some still may crave part-way actuation, and some may be unlucky with trying a particularly dirty/non-well kept example. But, I'm sure we can all at least agree there's some nifty legacy to these keyboards and we can all appreciate the fact these tried to provide a better alternative to the rubber domes that were entering force during the '90s.

The M4-1 and its RT3200 successor
The M4-1 and its RT3200 successor

Unfortunately from our 2021 perspective, we know that battle was eventually lost. Full-travel rubber domes are cheaper and less complex to make and scissor switches can be much slimmer, hence we seldom see sleeves switches anymore. These IBM Space Saver Keyboards were succeeded by the full-travel rubber dome IBM Space Saver II Keyboard (model RT3200) produced for IBM by NMB. And IBM just forgot about these keyboards. But, I want to end on a positive note. Clearly, these sold well enough in their heyday that examples of these keyboards can still be found. We know these keyboards were well regarded due to the reputation that the L40SX and early ThinkPads hold after all these years.

Hopefully, this article will continue to let people know that these existed and that they had a place in history. And, I hope this article helps teach people about this little snippet in keyboard history. Cheers, and big thanks to everyone that contributed!

Extra photos gallery

I've included some extra photos that were prepared for the article but I couldn't quite fit in or were found to be unnecessary. Some of these help explain earlier points, whilst others were just for viewing pleasure. Enjoy!

Further reading

External resources

Part/FRU number lists

My buckling sleeves keyboards


This article was one of the largest undertakings for the website since the start of creating the keyboard database. However, a lot of things had to come together to produce this well-illustrated and definitive article on the Model M4 and M4-1 keyboards. The following people deserve credit for helping to make this article possible:

  • aichi JAPAN provided photos and information on the IBM 5535-ZAD computer.
  • deskthority user arkanoid provided photos of the Lexmark-branded Model M4 keyboard.
  • Brandon @ ClickyKeyboards provided photos of the Key Tronic-OEM Model M4-1 keyboard and numeric keypad.
  • Dario @ Dario'sCope provided a photo of the US-made IBM Model M3 numeric keypad's rear sticker. I also discovered the M3's existence through his deskthority post regarding it.
  • Jack @ laptop.pics provided photos of the IBM PS/2 CL57SX and its keyboard assembly, and numerous Model M6-1 'birth certificates'. We also had several illuminating discussions about Model M6-1 ThinkPad keyboard assemblies that helped shape the ThinkPad keyboard section.
  • Troy @ Unicomp provided some interesting background information regarding the M4 and M4-1 from Unicomp's perspective. The codename "Surf" was discovered through this.
  • Uberman765 provided photos of the IBM PS/2 L40SX and its keyboard assembly.

Corrections & changes

  • 2021-07-07: thanks to deskthority user arkanoid, I've added some photos of a Lexmark-branded Model M4 in the "Lexmark, Key Tronic & Unicomp - a trio of OEMs" section!
  • 2021-07-15: in the "The architecture underneath the skin" section, I erroneously stated TrackPoint III was from 1997. The correct year is 1994, when the negative inertia transfer function was patented.
  • 2021-07-23: at the end of "Lexmark, Key Tronic & Unicomp - a trio of OEMs" section, I've added the beige Unicomp-made IBM Model M4-1 badge style to the IBM branding styles list. Thanks to KeebTalk user jhambone9190 for bringing this missing detail to my attention.
  • 2021-09-08: in the "Model M6 & the golden age of ThinkPad keyboards" section, I've clarified that the IBM ThinkPad 365X/365XD keyboard shown is a Japanese keyboard with Thai language stickers applied (by the previous owner). Thanks to deskthority user :Dön: for pointing this out from their Japanese perspective.