Comment by bsder

I mean, if you have the boards, then you obviously have the definitive answer about layers. Can't argue that.

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.