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The "FM" series probably made its debut in the late 1930s and were a heavy duty product line with larger coils and magnetic rotors and more robust caps. Here is some information with kind permission from Fairbanks-Morse to keep yours alive and well.
Here is some service information to keep yours well.
Here is a manual for the later version.
Here is a parts list. Many parts are made of unobtainium, but many good used and new old stock parts are still lurking and many can be made by stepping into the machine shop. The upside is that when these mags are in any state of repair they are hot and dependable.