The early Unix history of chown() being restricted to root

Summary

A few years ago I wrote about the divide in chown() about who got to give away files, where BSD and V7 were on one side, restricting it to root, while System III and System V were on the other, allowing the owner to give them away too. At the time I quoted the V7 chown(2) explanation of this: [...] Only the super-user may execute this call, because if users were able to give files away, they could defeat the (nonexistent) file-space accounting procedures. Recently, for reasons, chown(2) and its history was on my mind and so I wondered if the early Research Unixes had always had this, or if a restriction was added at some point. The answer is that the restriction was added in V6, where the V6 chown(2) manual page has the same wording as V7. In Research Unix V5 and earlier, people can chown(2) away their own files; this is documented in the V4 chown(2) manual page and is what the V5 kernel code for chown() does. This behavior runs all the way back to the V1 chown() manual page, with an extra restriction that you can't chown() setuid files. (Since I looked it up, the restriction on chown()'ing setuid files was lifted in V4. In V4 and later, a setuid file has its setuid bit removed on chown; in V3 you still can't give away such a file, according to the V3 chown(2) manual page.) At this point you might wonder where the System III and System V unrestricted chown came from. The surprising to me answer seems to be that System III partly descends from PWB/UNIX, and PWB/UNIX 1.0, although it was theoretically based on V6, has pre-V6 chown(2) behavior (kernel source, manual page). I suspect that there's a story both to why V6 made chown() more restricted and also why PWB/UNIX specifically didn't take that change from V6, but I don't know if it's been documented anywhere (a casual Internet search didn't turn up anything). (The System III chown(2) manual page says more or less the same thing as the PWB/UNIX manual page, just more formally, and the kernel code is very similar.)