I believe the M3 did sidecart the M4-1. If you pull off a keycap, it should have Lexmark's buckling rubber sleeves.
IBM has released a lot of similar looking devices, including the one I have:
This one is the IBM ThinkPad 560 Numeric Keypad Option (P/N 95F5446) from the later '90s. Instead of an SDL port, it's got a straight PS/2 cable, and it doesn't work, although the pass-through for the mouse does work. I've tried it with a standard PS/2 converter, Soarer's (mouse doesn't work with this), and on my ThinkPad 570 that also has a PS/2 port designed for this, and I've never been able to get actual keypad working. It does have a very bad 'electrical' smell though, so I think something's blown on the mainboard. Anyway, I think these keypads are pretty cute.
Anyway, as XMIT said, it should be simple enough to tack on a microcontroller to these things to get it working on a modern PC. Parasiting an internal Soarer's or making a circuit around the membrane itself like people have done to IBM Wheelwriter typewriter keyboards to USB mod them are valid options that could be applicable to one of these too. As for making it into a calculator, I believe Google should find you some good tutorials with standard membrane keypads that you could adapt. Cheap-enough Arduino 'starter' packs with the cables, LCD module, breadboard and microcontroller are available to get you started too.