Bill, I don't know when the diode appeared. I have noticed that later clocks often have the polarity written on the face, so those most likely will have a diode. Of course if the clock has had a previous restoration, the face might have lost the label or a non-diode clock might have had one added.

Incidentally, the more recent version of the clocks4classics upgrade is I think polarity insensitive. I don't know what the various upgrades consist of, but, for anyone who wants to cut costs and keeps the pin operating at a much reduced current, it should be fairly easy to design a circuit that uses a transistor (switched by the pin) and a couple of resistors to drive the movement.

 

Peter
Thanks for that information.
Cheers