Anything of particular note? Hmm... Definitely etched "in-house." I keep thinking its crazy there's such tiny power/ground traces, and that they meander like they do, but the vast majority of the board doesn't use them.

Working my way back into the motherboard, next, from here... That'll likely take longer, since the friggin traces jump sides so often...

Updated Keymatrix. Thankya to @ziggurat29 

Note that Col3/row3 has no button, nor a place for one, but @ziggurat29 says there's code for it(?!). I don't see any hidden buttons/pads on the keyboard nor mainboard. Though I haven't hunted those traces on the mainboard.

Oh Snap, I forgot to make note of the fact most each button has two names, which the "2nd" button selects. If I understand correctly.

....

These be the LEDs. If you don't care too much about the wiring, the 8255 on the right is where to look. The labels mostly correspond with the button names. Although four LEDs are not associated with buttons. "Pause" and "Stop" are the easy ones. "Slew" and "Scan" are both button-names and independent LEDs. So, I called the independent LEDs e.g. "Slew Active" and the ones on the buttons "Slew BTN"

They're driven at their cathodes through 74LS245's on the mainboard, we're guessing for drive-strength, but 245's don't invert, so in code I'm pretty sure, they're also active-low. The 245's DIR and OE pins are tied active.

There seem to be potential for two more LEDs which are N/C at the "keyboard" PCB. Again maybe suggesting the mainboard was intended to be used in other systems.

8255 cleaned-up, and in its entirety: