Hmm, from what I read, back in the day, how ever long ago that was, Mac's opened files according to the header file for determining the file format. From what I gather new Mac's support file extensions and maybe file headers as well.
So, going by that and knowing that windows psd files have both a header and the file extension and uses only the file extension, my guess is mac can do the same. Try creating a copy of the file and add the .psd extension to the end of the filename. If it works, it works. If not, at least you tried something.
Good thought - I did try copying the resource fork folder and the file to a mac, but didn't rename the file to a .psd, thinking the resource fork file would handle that. I'll give it another try. I DID rename the file to .psd on a windows machine but that did no good. Were there other .xxx extensions back then it might have been?
I tried opening the files as text, but didn't see anything obvious to help in this search to open older files.
Its possible the mac and pc versions were not compatible back then.
If the file is a layered file from photoshop, then it would have to be a psd (photoshop) file as versions before 6 wouldn't save layered files
in anything but the psd (photoshop) format.
On a windows machine if your adding the file extension or using File>OpenAs>PSD that should work on a good photoshop (psd) file, even one from photoshop 3 on an vintage mac, but
since infraview can't even open it, it may well be the file has been corrupted somehow.
On a later mac you should be able to use File>Open, highlight the file in the open dialog and then choose psd.
It might help if we knew what versions of photoshop you have now.