I have the small diametre generator with a chunk missing out of the pulley. It still works, but I have to change belts more often than I do my oil!
Yes, every vendor claims their later, non-threaded, keyed pulleys will work for a 1939 tractor, but if it has the original "small barreled" generator on it, they do not.
Tim Daley had a bunch of the correct style pulleys cast several years ago. Does anyone know who has them now?