V6 was "released" in May 1975, and that document was also published in May 1975 and says that the software had been running for about a month, so it's entirely possible that the ARPAnet Unix work was done before V6 (or perhaps they were an early test site: I don't know what the policies were around that).  It's been many years since I've read RFC681 closely and from my quick skim just now, I don't think they say what version of Unix they're running.  It's clear from the RFC that they had been running Unix for more than a month given the description of their site, and if I had to hazard a guess I'd say they were running V5; perhaps heavily patched.  I idly wonder if any of that work has survived; it would be interesting to see an ARPAnet/NCP implementation for early Unix.

But to address your question yes, Unix was certainly networked well before UUCP emerged in V7.

        - Dan C.