Originally Posted by
HillRider
Some nutted axles were 3/8" or 9.5 mm and, if that's what the frame originally came with, it may be the problem. A few strokes with a file should solve the problem but be sure to file on the lower face of the dropout (further assuming horizontal dropouts) to avoid alignment problems.
+1, especially on the reference to only filing the lower dropout face. However, rear dropouts not 10mm wre rare in the era when your frame was built, since they wanted to be able to fit a 10mm QR axle whether it was spec.ed or not.
Before filing the dropout open, check that it wasn't closed a bit in handling. Use a caliper or improvised gauge to see if the top and bottom faces are parallel, or if the slot tapers to a slightly pinched, or simply burred end. You can also remove the QR skewer and see if the end of the axle fits at the base of the slot.
If it's a matter of a bent dropout, you have a choice of flexing it back to parallel by fitting a wedge or flat bat and gently levering it square, or leaving it as is and filing it parallel. Either way you'll want to use the home made gauge checking as you go.