@SSTV2 : Wow, more surprises in terms of values. It's good we're making emulators, the only real way to preserve those old machines and technologies forever, but it's like no one is actually validating those emulators against real machines - not that I am blaming anyone, I myself was not able to buy real PS/2 with XGA card. That's why I am grateful there are people like you that are willing to invest some time and help. Obviously, just reading the specifications is not enough.
So, COMPID/PS2BUS test is OK, as far as it detects MCA bus, it doesn't matter it doesn't recognize the Model ID - that I guess is for hardcore PS/2 fans, i.e. further to work and build full table with all Model IDs that IBM used over the years, if they are interested in such a thing.
I can see in x86 ASM code of disassembler-ed IBM's XGAAIDOS.sys, it compares Index 53H value to 0x30, but I did not expect it will be exactly 0x30 and it's good on 2 machines is the same value - I guess we just hardcode it as it's not documented what it means - maybe if someone else makes dump and we get different value than 0x30 we can eventually understand what it means, but for the time being, just hard-code it to 0x30 should be fine. (I was suspecting it's some Revision value, but that no longer makes sense, because you get the same on XGA-1 and on XGA-2 card).
Now, the new main question - what you get for Index 52H is confusing, even for a moment I thought you maybe swapped the values from XGARD52H and XGARD53H output, but I don't think that's the case. So, please, get XGAKIT:
and run "XGADEMO.EXE" on the machine with "XGARD52H.EXE - Index 52H value: 21EF;" - I don't see any way how it can work with value of 0xEF for Index 52H and so I expect it will error out - in any case error out or not after quit "XGADEMO.EXE" , run again "XGARD52H.EXE" to see what is the current value there. For completeness and maybe even more surprises you can do that same test on the other machine with "XGARD52H.EXE - Index 52H value: 21EA;" - at least 0xEA seems like good value that "XGADEMO.EXE" will run with no complains, but who knows...