PLUS it turns out the drive we're trying to mount uses Veritas/VxFS, which is a commercial-only filesystem. Interactive Unix 4.1.1 doesn't have Veritas by default, you'd need to buy it separately. back in 1998.
and if your reaction to that post is "BUT FOONE, LINUX HAS VXFS SUPPORT!"
yeah, no, it only supports from Veritas version 2 onward, and this disk is version 1