There was V8, V9 and V10 in between, with V8 using BSD as base, iirc, but creating streams and TLI/XTI instead of BSD Sockets (XTI was superior API, tbqh)
TLI/XTI are from AT&T SVR3/SVR4, not Research Unix (V8, V9, V10). dmr's V8 streams were the basis for AT&T SVR3/SVR4 STREAMS (all caps!), but quite a bit simpler: https://en.wikipedia.org/wiki/STREAMS.