Wholeheartedly agree with the observations on forgotten versions, lack of source and a smaller pool of people back in the day. It is not just the Research versions, also the internal AT&T versions and the base System V versions get little attention. Same reasons I think. Luckily, these days the sources are available (although in the case of SysV of unclear legal status). Part of the problem I think is that it is not well known what innovations are in each version. About 2 years ago I did a lot of spelunking through the V8 source and with the help of this list could come up with a list of highlights for V8 (text is now on the TUHS V8 source web page). Never had the time to do that for V9. I think it was mentioned that it had a new filesystem with a bitmap free list. Also, it seems to have a lot of cleaned-up implementations of things that were new and rough in V8. No clue what was new in V10. Similar with Unix 3, Unix 4 and Unix 5. I’m thrilled that the docs for Unix 4 showed up recently. In these doc’s there is no material on IPC. From this I think that the IPC primitives from CB-Unix did not get merged in Unix 4, but only in Unix 5 (in a reworked form). Personally, I’m still working (off and on) on recreating the Reiser demand paging system. To keep it interesting I’ve now got Sys III running on a small RISC-V board, and when I find another time slot I’ll try to add Reiser paging to it. So the forgotten versions are only mostly forgotten, not totally forgotten :^)