This starter switch problem surfaced several times years ago with many complaints on Forums and complaints to parts suppliers.
Only one (1) manufacturer was producing them, i.e., they were all made by "Vintique Inc.", (Made in U.S.A., A-11450, 1928/31 Starter Switch Assembly).
The "only" two (2) problems I found was with the top sheet metal body part which had a stamped round opening that was:
A. Far too tight fitting for the moving rod plunger; and,
B. this stamped opening was provided at an incorrect rod angle which was much more in the vertical position than that of the original rod angles on original starter switches.
When the starter rod was depressed, the plunger would get jammed and stuck in the downward position where the starter continued to operate after the engine started; hence, not enough clearance for the plunger to move up and down with the springing action of the non-ferrous bar hinged on the electrical connection bolt.
I elongated and reshaped this round opening in the top sheet metal body making it far more oval shaped to allow the moving threaded round plunger to move very "freely" and to be positioned at the same angle as the "moving" starter rod.
I still have the another new starter switch, same manufacturer, unwrapped and not modified ..... with the same top sheet metal orifice problem; i.e., the top body hole was incorrectly stamped where the plunger angle is too far in the vertical position, and the hole is round as opposed to oval and far too tight.
After this top body sheet metal modification, it has been working well & problem free for the past 10 years.
Hope this helps someone needing a new starter switch who may not be able to find original starter switches in their neighborhood.