@Prof Shadow:
Thanks! While I do have Vols II and III of the BSP's (again, as bad xeroxes.. a pile of binders about 5" thick), and recently located Vol I on cowboy's BSP site (wish I'd had that 10 yrs ago!), this is again a much nicer version than I have.. and having text-searchable PDF versions is waaay better. Also, wow, there's /color/ in some of these documents? (pp.53 of that pdf -- red arrows showing the path to ground for lightning). I should probably toss my ugly xeroxes in the recycle bin, along with the silverfish that are munching on it, and just depend on the PDFs at this point.

While it does say it's a "service" manual, as Arthur points out, there's limits on how far the BSPs go into detail. The BSP's seem to be "field service", as opposed to.. hmm, what's the term, "engineering" servicing, where e.g. dead line cards or burned out ComKey mother boards would go for servicing, to determine if they're salvageable or blown beyond repair.

@Arthur:
Thanks -- the "Bell System Technical Journal" seems like a very good lead -- that's a big rabbit hole to go down, great! Excruciating detail indeed; some of that stuff is like reading Scientific American; heavy on the math and physics of electronics and telecom.

Apparently IEEE and Nokia have teamed up to make these available as PDFs.. looks like that's a "join and pay" situation to access the articles, at least from what I gather on the IEEE page. Seems there's other providers as well. Would be interesting if they have articles focused on business phone systems and key system design/architecture. I'll sniff through the abstracts.

But I also found what I think is a "free" a way into the documents via archive.org; there's a page a bit easier to browse there, titled The Bell System Technical Journal (1922-1983); an "infinite scroll" of thumbnails of the first page of each article.

IIRC, there was a guy from England (?) here on the forum some years back who was interested in studying 1A2 design, and sounded like an EE guy who wanted to go down a similar path to the one I've been taking. I think that thread might be locked, but he might be interested in those BSTJ articles as well.