There is a huge demarcation up to and including 4.1BSD vs. anything afterward. .... (lots of great deal trimmed)
I remember being at small software company (
mirror.tmc.com :) and we bought a Vax750 and mtXinu came and supported 4.1c BSD on it (Ed Gould himself), and then we got upgraded to 4.2 when it came out. I remember waiting on tenterhooks for the upgrade. And having my mind blown when I typed in the "network programming" exercises and found sockets a way for two unrelated processes to communicate.
Unless my memory's wrong about the 4.1c/4.2 thing. I think we had an emacs tape (CCA? Unipress? Not sure) that needed 4.2 and we just had to wait for the supported version.