E-Pay
I'm sure that's the same oddball size I have on the axle mounts for my 1944 and 1950 M19 ski/wheel trailers.
I wrecked a couple getting them off (to preserve the stub axle) thinking they would be easy to find, but in the end I just had to get them from E-Bay, took a while, cost a bit more than I would normally pay, but since it was an axle securing nut you're not going to take chances with it.
Gordon_M