The PCB construction is curious (which you say is multi-layer) - why use a grid of 0.1" holes? Is that so it could be easily jumpered? Can you tell if the traces run through the holes or between them?
I don't have the patience to reverse-engineer these types of boards, but I do find them really interesting to think about. CAD was just getting started (I just looked up that Gerber format was released in 1980) so I wonder if the masks were hand-drawn.
When you say that the 125 added memory management, what does that mean a little more specifically? My guess is either PDP-11/z280 style paging without page tables (the 16 bit address space makes just having enough IO registers to cover the address space tractable) or some simple segmentation hardware, but it'd be neat if there was another hardware object capability system I didn't know about.
I don't have the patience to reverse-engineer these types of boards, but I do find them really interesting to think about. CAD was just getting started (I just looked up that Gerber format was released in 1980) so I wonder if the masks were hand-drawn.