I would look in the git repository for Unix: https://github.com/dspinellis/unix-history-repo. It isn't guaranteed to be correct, either, but should have a commit describing its addition that can be trusted slightly more.

The earlier i can find there without digging deep is BSD 4.4 Lite, a mega-commit by Rodney Grimes (https://github.com/dspinellis/unix-history-repo/commit/d2273...)

Being an enormous commit, changes are that it merged in existing stuff.