WetGeek
As I thought, GIMP wouldn't admit to knowing anything about these files, either.
I even tried opening them on my Windows 10 VM, without success. I know for a fact that it's a Windows format, but apparently one that's been deprecated, probably long ago.
Even if nobody knows for sure what would work with these files, I'm open to suggestions of what might open them. GIMP and Gwenview are about all I know of when it comes to Linux imaging applications. There are probably others, one of which might work. Maybe a conversion utility that might change these to a more common format?
Thanks in advance for any help with this. It's hard for me to believe that accessing digitized photos could be so problematic, even those that were digitized 25 years ago.