Honestly, I found this utterly fascinating for some strange (fetishistic?) reason.
When I get home, if I remember, I would like to grep through four additional large and source-available and influential older code bases that seemed to have escaped mention: IBM MVS 3.8, Xerox UTS/CP-V, MIT/GE/Bell/Honeywell/Bull/etc Multics, and UM MTS (Michigan Terminal System).
These all should predate BSD in many cases by decades and might well have earlier references.
Maybe someone will beat me to it.
I had a look through MULTICS, didn’t find anything plausible. The other three sound like they’re worth a shot :)
(MULTICS was a bit of a pain. Also AFAIK the only public source for it is a late-stage source dump with no versioning history; so it’s hard to say with any degree of confidence when any specific part was written).
There are actually two releases, if I recall correctly - an unofficial “leaked” one that originated from MIT, and the officially blessed one from Bull.
While neither release contained modern revision control, major edits and changes were always described in the comments at the beginning of the file, so you are right, it is of little help here; unless you could discover an “XXX” comment referenced in a source file with a pre-81 header — but even then it would likely be impossible to prove the edit date definitively.
I think the sources for the other older systems I mentioned resides on SHARE and Bitsavers.
For posterity, there wasn’t anything of great interest in these codebases.
@jsnell have you asked Bill about it?