I concede, Barit1, you're "bang-on", as regards that "throttling back". Learned it the hard way on numerous winter launches, wlaking unerneath an engine pod, when the guy at the throttles, pulled back. But, I would submit, regardless of the specific reason why(!!) the front stages are feeding the rear ones more air than they can handle, and even with just one spool, the concept still stands.
Also, given the specs cited here, for the J-91, I would bet that the "monster turbojet" seen, without a placard, may well have been a surviving example, which for superficially resembling a J-58, except for sheer size, could have been confusing.
Would have loved to have been there and seen it myself.
In any case, thanks to all the great minds assembled here for the superb info re my questions, and does anyone know of a book containing some of this stuff in print so I don't have to keep pestering you?
BTW, has anyone noticed that the basic J-58 "gas generator"actually looks like nothing more than a J-75, with the high pressure compressor missing, and the N1 attached directly to the diffuser casing?
Last does anyone know if the J-91, carrying after all, the "in-house" designation of JT-9, was in fact the core for the high bypass ratio JT-9D, and if not why did P&W use the number twice?