I've been reading Exhibit 14971 from US vs. IBM (Parts 1, 2, 3) which seems to give a very good overview of the history of the computer industry up to 1980, particularly the way IBM handled its product planning beginning in the '60s.
One thing that puzzles me though is the "gap" in the mainframe naming conventions. We have "System/360", "System/370", and "System/390", but between 1977 and 1990 IBM used the 30XX/43XX/93XX model numbers as designations.
Why didn't IBM market these systems as "System/380"?
I can sort of understand 1977 being a little early for announcing a series "for the 1980s" so the dates don't line up as well as with the /370 and /390 families, but were there other considerations in play?
At the same time, the 30XX/43XX series was not advertised as "System/370" per se but as "System/370 Compatible". This choice puts a deliberate (albeit small) amount of separation between these machines and the rest of the series. Why create that distinction?
Are there any sources available that point to IBM's reasoning for these choices?