Small stuff like old numeric keypads inevitably gets overshadowed when 122-keys and SSKs are all the rage. As a result, when I was on the search for technical details about this device, I found basically nothing. Obviously, I was going to have to discover these details myself, and of course, I'm happy to show you all in the process.
This was the numpad that played sidecar to the Model M4 (without TrackPoint) and Model M4-1 (with TrackPoint) IBM Space Saver Keyboards. Not to be confused with the Space Saving Keyboard (SSK), the Model M4 series was the desktop adaptation of the integrated keyboard found on the IBM Personal System/2 L40SX laptop. By the mid-'90s, the M4s and these keypads were referred to in most ThinkPad hardware maintenance manuals as an order option. They were manufactured for IBM by Lexmark, Key Tronic and Unicomp at various points in the '90s. Lexmark and Unicomp even marketed their own branded versions, with Unicomp specifically naming their M4-1s the Unicomp Mighty Mouse.
Examples from Lexmark and Unicomp feature Lexmark's excellent buckling rubber sleeves switches, which are in my opinion the best rubber-based switch technology to come from IBM and co. Many early ThinkPads also featured these switches too. Unlike rubber dome switches that by their very nature use the dome as a combined tactile, return and actuation element, sleeves only use the rubber component as a tactile and return element and use a separate rod on the keycap to bridge the membrane contacts. In Lexmark's implementation, this eliminates the mushiness feeling on bottoming out that is oh so commonly associated with rubber domes. The weighing of these switches is roughly equivalent to membrane buckling springs'.
In terms of external details, we get not much. The device is of a very clean and utilitarian design that is obviously designed to conform to the design language of the full Model M4 and M4-1. As such, it features the same flip-out feet design as them. Which is a very good thing since they are excellent. The rubber padding on them is very grippy, held up well over the years, and quite frankly superior at stopping the device slipping around compared to a standard Model M's non-rubber feet.
Around bottom, we get your typical Lexmark-era Model M sticker. It's detailed and to the point just like any other mainstream Model M sticker from IBM, Lexmark or Unicomp. One thing I can't explain is why it's designated "M4-1" since that specifically denotes the TrackPoint-equipped host keyboard - would it not make sense to instead call it Model M4 since this is intended to complement both M4s and M4-1s?
Pulling off a keycap reveals the mounting system, which is a bit more involved than buckling springs' one. The two hooks on the sides clip the keycap onto the board, but the cylinder at the back doesn't go into anything. I assume that its only purpose is to help prevent the user from damaging the membrane via the keycap's rod with too much force. Speaking of which, the large cylinder in the centre is what pushes on the membranes.
As you can see, buckling rubber sleeves switches resembles an upside-down rubber dome that is exposed and plays no part in actuating the switch.
You may or may not be surprised to learn that the simplicity of appearance continues inside too. In fact, this keypad is completely brainless. As you can see, the assembly is a minified version of a typical Model M assembly - a membrane sandwiched in between a barrel plate and metal backplate held together by rivets and connects to a PCB via Triomate connectors.
Getting closer to the PCB, you can see what I mean by 'brainless'. It is just two 5-pin Triomates routed directly into an ethernet-like jack with a jumper in between. Note that this is not an modular 8P8C ("RJ-45") jack, rather a modular 10P10C ("RJ-50") jack. The jacks are physically-compatible but not completely electrically-compatible with each other's cables. As a test, I tried an 8P8C CAT5e ethernet cable to see what happens - basically, part of the membrane simply doesn't register due to the missing pins.
The traces confirm the simplicity and show what lines the jumper is wired on. As of the time of writing, I still have no idea what the jumper does. There's a similar jumper on the M4/M4-1 keyboard's controller around its RJ-50 jack. I've tried operating the keypad with and without the jumper, but there seems to be no difference at all. If you know the purpose, please get in touch with me!
And that's about it. Hopefully, you enjoyed today's little exploration of this keypad. I know that the Model M4 series isn't exactly glamorous, but should you find one of these and it's host keyboard for a decent price, you could find yourself with a capable quiet but tactile keyboard!
But before I go, I'll leave you with this cute photo of the this keypad with its larger sibling!