This machine uses a Pentium Pro processor and is specifically model 8639-E80. Museum surfer Mike Baldwin offered some insight into a similar machine that he had:
This was sold by IBM as a P/390, and because it has a mainframe
board installed (with its own 128 MB RAM), it is capable of running an old mainframe
operating system called OS/390 (and can also run VM and VSE operating systems).
Actually it is also technically capable of running older releases of a modern
mainframe operating system such as z/OS, but that is illegal.
The newer but expiring system is an xSeries server running Linux or Unix, and mainframe
emulation software. The expiry is due to very messy legal problems
between IBM and a couple of other companies.
Basically you can only get an IBM mainframe from IBM now, and there is no
"personal" size (although real ones are only the size of one or two server racks today).
Before the legal problems, and actually on a few remaining systems, you could
run a mainframe operating system on a ThinkPad.