NCR UNIX is a spin of SysV UNIX. So it should be a Sysv filesystem (I think).
I noticed you modprobed the kernel mod instead of compiling SysV/Coherent FS support into the kernel directly. Dunno if that has anything to do with it.
A couple of things then....
Maybe if you pick up an old distro with a 2.0 or 2.2 Kernel, do a quick install, and try that, or maybe check with NCR (or whomever they've become now - prolly will not yield much though).
The last thing I can recommend to you, if your data is important enough for you to spend the money, is to take the disk to Data Mechanix in Irvine, California (
Data Mechanix | Data Recovery).
Their phone number is: 800.886.2231 and although it's been many years since I've had to have a clean room pull data from a drive, they always did me right, and I believe the owner's name is Craig - but don't quote me on that.
If you get him on the phone, he might even offer you a couple of suggestions as to how you might mount it.