I'd expect a UNF thread if it's made in America............... though having checked my (British-made) Stanley 50s from the 1950s / 60s have what is close to a unified thread on that wing-nut, though being British from the pre-metric age, it could be BSF. I have just counted 27 TPI, so allowing for my eyesight ........... ( that could be 1/4 BSF, which should be 26 TPI).
There may have been some local variation during the manufacturing life of this plane, depending where it was made. If I recall,I'm not sure that the pitch on UNF is exactly the same as BSF...... slight variation.
Ideally a 1/4" die of each size run onto the bolt would determine which is which.
Sorry can't be more precise - Hope this helps.
Good luck finding some. Perhaps a metal-working member can run something up......?