The problem is probably the position of the solenoid. It's adjustable, as evidenced by the slotted holes in the mounting bracket. The correct procedure for installing it is to push the exposed linkage into the "engaged" position with the battery cable disconnected and move the solenoid into position so the contact disc inside is engaged, then lock the mounting bolts down. The pinion gear should be fully engaged in the flywheel before the starter motor ever begins to turn. Due to manufacturing tolerances, sometimes it's necessary to elongate the slots in the solenoid mount in order to get proper synchronization of the gear engagement and motor start timing. This adjusting procedure should be done (and tested) on the bench before the starter is installed.
Once again- - - -stuff made 50-something years ago was not always manufactured perfectly, and the junk made in China these days has even less quality control. Expecting every part we buy to fit perfectly without a little massaging probably isn't realistic. A real mechanic makes things work- - - -a wrench jockey swaps parts and hopes!
Jerry
