[GlibMonkeyDeath]

>As you point out, if they designed this thing in the late 1970s, there is no >reason for those giant arrays of drill holes.

Yeah, this was precisely what my original question was about. Weird claims in this thread - we had multilayer boards with CAD layout in 1980, but can't drill through-vias in anything but a complete grid pattern? These are low-volume production boards too, trivial to drill an arbitrary pattern. (Edit: Obviously we did have multilayer CAD designed boards in 1980...)

My own (tongue-in-cheek) guess: someone prototyped this circuit on a perf-board with point-to-point soldering, then handed it off to a too-literally-minded junior engineer to do the PCB layout :) Unless there is some aerospace thing I am missing for having the full hole grid available...maybe they thought it was a weight savings?