Yes, power switch on the back was OG AT PSU, but not very common after 89 or so. Have mostly only seen it in 5170 lookalike styled AT cases after that, which were still selling a couple of years into 90s, dunno if old stock or they still made them until then.

Edit: probably easier to use a switched ATX with PowerGood jumpered and motherboard power adapted to AT than finding one... Some 25 years ago, there were a few NOS around dirt cheap because nobody wanted them for more recent cases.

EditII: if you do find one, double check size, they came in original AT size which was huge and a more compact version.

Thanks for the info, that's about what I figured. I don't know that I've ever seen an AT power supply in the size that apparently came in this case, given the screw hole pattern, so I'm not expecting to be able to find one. I will likely just pick up a standard AT power supply and retrofit it in the case and route the power button to the front of the case, somehow. This should be interesting...