I'm pretty sure it was in 4.1. It shipped as a binary only, IIRC.
That might be right. At Tektronix, we had it one V7 on a PDP-11 in the late 1970s (before we were running 4.1), and I know we had sources but I have no memory of the provenance. It was likely they came from a UCB person working there in the summer (Mark Bales, Jim Kleckner or one of the CAD folks would be the prime suspects).