It's both the Tru64 code and the "gen 2" HP-UX port. See SF download page.
And yes, I misread your original post as saying that AdvFS was originated in HP-UX. And yes, I'm well aware of how Compaq and HP screwed up DEC and pissed away some great technology. Of course, DEC did a pretty good job of screwing itself. Great engineers; incredibly piss-poor management.