Sears and Roebuck lathe model 101.06242

Hello, I have been reading a little about my lathe on the web. I have a question about the spindle. I have measured/counted the threads and it seems to be 3/4" X 16 TPI. However, I have a chuck and some other accessories that I KNOW are 3/4" X 16 TPI and it won't thread on - some pieces will go on a few turns and then they stop, some won't go on at all.

Would there have been an odd or different thread count for this model? Am I just missing something that is stopping my chuck, etc. from going on?

By the way, I have two homemade faceplates that came with the lathe and they screw on perfectly. They are made from a large nut welded to a ring of 1/8" thick metal, and work pretty well for small stuff. Any help is much appreciated.

This was my grandfather's lathe in his farm shop, and I inherited it after he died. He probably bought it new, considering he was born in 1899, and was the kind of person to repair and maintain things forever. Thanks again!