Version 7 Unix[1] (the first Unix I ever used) was also small enough to understand well. Since we ran it on a PDP-11/45[2] that had only 256K of RAM and the root file system lived on a 2.5 megabyte RK05[3] disk, it had to be pretty small and simple. We even had the source code for it. I've held on to an ancient copy of the printed documentation for it all these years. I think it's still my favorite OS.