I think it's System V Release 3.0 or thereabout
Basically the same machine was also sold as the 3b1; the difference
between the 7300 and the 3b1 is that the 3b1 has room for a taller
hard disk drive.
I think the OS would best be characterized as SVR2 with the addition
of the 4.1bsd VM system ("real" SVR2 had no demand paged VM) and the
further addition of its own unique approach to shared libraries.
It's definitely not SVR3; no "STREAMS".