There was only one platform designed to be POSIX in all respects, the OSF/1 UNIX for the Alpha processor from Digital Equipment Corporation.

It was first known as DEC OSF/1, then Digital UNIX, then Tru64.

Previous to OSF/1, DEC had sold Ultrix, first on VAX, then on MIPS.

https://en.wikipedia.org/wiki/Tru64_UNIX

The wiki says that OSF/1 was first released for MIPS, but that was before my time, and it wasn't supported for long.

I did use Ultrix on MIPS DECStations in college.

https://en.wikipedia.org/wiki/Ultrix