That said, when the distribution of UNIX moved to USG in Summit, things started to a bit more formal. But there were still differences inside, as we have tried to unravel. PWB/TS and eventually System x. FWIW, BSD went through the same thing. The first BSD's are really the binary state of the world on the Cory 11/70, later 'Ernie.' By the time CSRG gets stood up because their official job (like USG) is to support Unix for DARPA, Sam and company are acting a bit more like traditional SW firms with alpha/beta releases and a more formal build process. Note that 2.X never really went through that, so we are all witnessing the wonderful efforts to try to rebuild early 2.X BSD, and see that the ephemeral nature of the bits has become more obvious.
I'm rebuilding 2.11BSD as released, not any of the early bits... :) 1991 is quite late in the 2BSD timeline (oh, wait, it's still going strong in PiDP-11 land).
Having said that, though, 2BSD through at least 2.8BSD gives the feeling of the tape of the day club. If you look closely at what's in the TUHS archive, and what's in Kirk's archive as well as other copies around, you'll likely notice small variations. Or you'll see a dozen or two files having newer dates than the documented release date. And the 2.79BSD tape... I'm more than half convinced it was really the 79th tape that had been made and they said 'nuts to that, for a while we'll do 2.8BSD since we now have a kernel'. This is pure speculation, I've not asked around...
2.9BSD, 2.10BSD and 2.10.1BSD all seem to be a little more controlled, though 2.9BSD has a lot of forks and it's not entirely clear they all started from the same spot. There's references to 2.9-SEISMO and 2.9.1 and 2.9 with patches and it isn't at all clear if these are the same thing or different (I think the same, but there's a 2.9 from princeton that's clearly a rollup release years later in kirk's archives).
And even my 2.11BSD reconstruction shows that proper CM wasn't deployed for it. I've found half a dozen missing patches that were not released as real patches, but showed up in the 'catch-up' kit that seems to be hiding these sorts of minor sins in the first couple of years after 2.11BSD was released. I'm down to 10-20 files that I'm unsure about ever recovering. These are clearly local files (different kernel configs, UUCP data, games high score files), and I doubt I'll be able to recover them completely.... Though in the scheme of things, they likely are the least important files since they only had relevance to the site making the tapes and were deleted from later versions (which is why I can't find them :).
In a way I've started thinking about this like quantum physics. Why you look at it at the macro level, it's all predictable, orderly and makes sense. But when you zoom in too much to any point on the timeline, you find that things get messy, chaotic and a bit indeterminate.
Warner