Interix wasn’t really a “replacement” - the fundamental architecture hadn’t changed, PSXSS was still there. From what I understand, Interix was just an enhanced version of the original POSIX subsystem code, not some from-scratch rewrite.
I thought that Softway replaced the Microsoft PSXSS with their own but I don't remember for sure. This would be an interesting software archaeology rabbit hole to go down.
Interix 2.2[0] shipped with its own PSXSS.EXE. I don't have a contemporaneous version of the PSXSS.EXE that shipped with Windows at that time to compare handy.
Finding a copy of OpenNT prior to the Microsoft acquisition is proving to be difficult.
Wasn’t their updated version a modified version of Microsoft’s original code, which they’d licensed from Microsoft? If that’s true, their PSXSS was more an enhancement than a “replacement”.
That would be interesting to know. I’d love to see a copy of OpenNT. Sadly, my experience started with it when it was already Interix. I know Microsoft negotiated source access/licensing with various third-party software “manufacturers” (like w/ Citrix for WinFrame). I’d love to know more about the arrangement with Softway.
However (putting aside the legalities of downloading "abandonware"), they won't let you download anything unless you first upload something they don't have. Heck, I probably do have something they don't have in my box of old floppy disks (which I keep on telling myself I will image one of these days), but my curiosity about this topic isn't strong enough to motivate me to do that.
[0] https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem
[1] https://en.wikipedia.org/wiki/Interix