A while ago I got a stack of basket-case mechanical keyboards that I've been restoring. This one is a Datacomp DFK-555 or maybe 86-43 according to the PCB. I can't seem to find a schmatic or a good photo of the inside of one of these online. Does anyone have one & want to pop it open, or can help me answer some questions?

For starters, I need to re-attach the cable. I have what I 95% believe is the original cable, but I don't know the pinout for the header on the board itself. The pins are just labelled 1-5 and the wires in the cable are color coded. There's also a ground wire that's not part of the cable, that was cut:

There might also be some missing components. On the back, U7 doesn't look like there was ever anything soldered there, but it does look like there was a voltage regulator removed at J4.

There are a few empty spots on the other side, too (R12, R13, and a spot for a piezo buzzer), but like U7 it doesn't look like the solder pads were ever used.

Here's the innards from the front & rear, if it helps:

So, TL;DR questions:
- what is the pinout for the cable header?
- do I need a voltage regulator at J4? If so, what voltage?
- does it matter that U7, R12, and R13 are missing?
- where should I hook up the ground wire to?