SCO Unixware was one. Quite a few "real" UNIXes. We had a mutli-processor 386 in high school that ran "real" BSD.
As you say, also SCO OpenServer will run on that dual pentium-1 board just fine. It's System V Release 3.2 based, has TCP/IP, SMP support, an optional X11R5 GUI, and all the unix bundle (with the original true blood Bourne Shell, also).