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.
That's what I was curious about - otherwise there would be effectively be through-vias along the traces (so traces could be probed between devices.)
The claim is multi-layer, but I seriously doubt that. I suspect that these are two-layer boards.
And if that's the case, the pattern is most likely because the holes precede the etch. And possibly precede the copper deposition so that the copper deposition can coat the insides of the holes.
And the holes are in a regular pattern because CNC simply wasn't a thing yet. You probably had some fixed array of drill bits that were used to make the holes in a very strict fixed automation fashion.
In addition, 2-layer has some big advantages over 4 layer for reliability (won't delaminate under launch vibration, for example)--which is an issue in aerospace.
And, to my eye, these boards simply don't look like the have 4 layers nor are the laid out like that: https://upload.wikimedia.org/wikipedia/commons/7/7a/Mitra_15...
Besides, even if it were 4 layers, the issue is still that drilling holes in a non-regular pattern simply wasn't something that could be done easily 1975.
