Lots of recaps of commercial unixes say stuff like "IBM AIX is nearly as old [...] originating for the IBM RT PC in 1986, though early versions also ran on PS/2 systems and System/370 and /390 mainframes. AIX started as an early port of UNIX System V [...]" (oldvcr.blogspot.com/2023/11/the-apple-network-servers-all-too.html) giving the impression that there is a piece of software with a coherent lineage there