Focus FK-5001 that I cleaned up and converted to USB. White Alps, original double-shot ABS caps. The calculator works.
EDIT: This fucker developed a short within like an hour of my closing the case back up. Now I have to either use my multi-meter to learn the entire 130-key matrix, or I have to desolder all the switches and kill the calculator function to turn the board into a hand-wired custom, though that would allow the left side function keys to be different from the top-row ones. Sigh.
Damn it! From a historical preservation perspective I hope you try to restore the original electronics, but I realise that could be a lot of hassle. Good luck whatever route you choose!
I have hunted and hunted for the short, but while I know exactly which keys are affected, I can't tell why. One interesting development is that while working on another board I plugged the Focus in to check a USB converter, and it worked okay for a minute or so. That makes me wonder if I might get away with simply replacing the capacitors, as I know several of them are for cleaning up the signal. Focus's engineers were bonkers AF with their value-engineering and never met a kludge they didn't like, so as an English major with a soldering iron, who knows what hijinks they were up to. I didn't see any tell-tale burnt-out caps, but the fact that "resting" helped even briefly is interesting. I may try that before I murder the PCB.