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.
As you point out, if they designed this thing in the late 1970s, there is no reason for those giant arrays of drill holes. PCB design was definitely past this point by then and it would have been a hideous waste of time drilling all that just to fill them all up with wave soldering. It also blocks your routing terribly.
However, I assumed that this was likely a port of something from much earlier given the enormous lead times that aerospace requires (especially in the 1970s). There is absolutely no good reason to leave those extra holes which can become an assembly mistake otherwise.
"The Mitra 125, sometimes called "Mitra 15M/125" succeeded the Mitra 15 in 1975" That is the design that got used for the Spacelab Metra 125 MS in 1980, right?
I presumed that this was a port of a board which was a port of a board which was a port of a board given that design was obsolete even in 1975 since they apparently switched to the AMD bit slice processors even that far back.
And looking at Altair 8800 boards, you can see that the landing pads were very much NOT trivial, and look like they might even be hand drilled given the poor registration. Excellon/Esterline machines were still not that common outside of very high volume in 1975. By the time the Apple II came online a couple years later, though, the Excellon drilling machines were pretty commonplace.
I imagine it was a function of the design constraints in that time period, and similar program needs.
There are very few records of any of this out there, similar to the Spacelab equipment I imagine. The -484 ran a Harris H100 minicomputer with an HP terminal and a GPIB bus. I've linked one of the few photos that you can find, but it's from an extended version of the test bench and has the minicomputer rack off to the right cut off.
Sorry for the off topic, but it's always cool to see stuff like what I spent years working on come across this site!
https://api.army.mil/e2/c/-images/2007/02/12/2612/army.mil-2...