Goodies! And totally unexpected ones at that.
The PC-X is a PC-D with an OMTI 5300 HD/Tape Streamer controller in place of the OMTI 5100 in the PC-D, which only has a HD connector. It runs Sinix.
I know one video showing a PC-X boothttps://www.youtube.com/watch?v=P0UQ-jrM_bw
but the guy (understandably) wouldn't dump the HD of his working machine, and he doesn't have the installation media.
I put out a call for help on de.comp.sys.unix for Sinix for the PC-D/X and was contacted by someone who knew someone who might ... you know those stories.
Turns out, the guy in question replied to my Email, still had original! installation disks and was willing to mail them out to me:
Not one, but two different versions
Happily Sinix was distributed on Floppy disks, and not, as I had feared, on a boot disk and streamer tape.
It would be great to get the harddisk on the PC-D going, so we can try to install Sinix. Some sources say, the -X variant has a different BIOS from the -D, but we'll see