V7 is the one that Andy Tanenbaum regarded as the archetypal UNIX in the MINIX book, and the original MINIX was, I believe, intended to be compatible with V7 (later MINIX aimed for POSIX compatibility). The most surprising thing for me was discovering that the whole of V7 was around 15KLoC. If you wrote it in a more modern language you could probably shave a good 25% off that, possibly more. For comparison, the bc utility in a modern *NIX is around 20KLoC.
It was of a level of complexity where a single person could read and understand all of the code and where it was quite feasible for someone to do a complete reimplementation.