https://en.wikipedia.org/wiki/Research_Unix
The term Research Unix first appeared in the Bell System Technical Journal (Vol. 57, No. 6, Pt. 2 Jul/Aug 1978) to distinguish it from other versions internal to Bell Labs (such as PWB/UNIX and MERT) whose code-base had diverged from the primary CSRC version.However, that term was little-used until Version 8 Unix, but has been retroactively applied to earlier versions as well.