I could do also with some help on this very same question. I too just picked up a lovely NOS frame from the early/mid eighties and the rear dropout spacing is only 100mm on that too.
I get that this isn't common but were some frames built this way or is it simply poorly built.......hence NOS?
If it is just an uncommon set up how do you get the rear wheel to fit and stop the freewheel catching the chainstay? If I remove all cone spacers it'll go in wit the freewheel's then too wide. Running Suzue hubs.