I recently got a Davall DTM-40 teaching machine which was made in England around the 1960's. There is practically no information about them however I found a few patents which explain their purpose but other than that, nothing. The thing weighs just over 3kg with the cable included. Considering it only has 21 keys, this is quite possibly the heaviest number pad I've seen...25-conductor wire (nearly 12mm diameter) with a HUGE 25 pin plug on the end, wow.
I thought the outside was impressive but I was pleasantly greeted with some logic boards once inside.
The patents stated that the keyboard would 'electronically lock' which is what I assume the right side board handles.
Interestingly, the left logic board uses a PCB edge connector rather than being soldered. This was probably so that the board could be swapped.
The keyboard is possibly the least interesting part of this. It uses Micro Switch magnetic reed switches and doubleshot purple keycaps. Pressing any key even slightly off-center results in binding. The larger caps are stabilized, but poorly so. I might prefer the feel of my Clare-Pendar reed switches.
I'm tempted to trace the PCBs and see how the thing works but it's unlikely a Teensy will get it working. It would certainly be interesting to have it on the desk as a numpad - 'Davall unsaver' has a nice ring to it