They both should work. I have a 1939 Allis Chalmers B that has an FMX4B magneto on it, which was definitely a replacement as the original magneto should have been an FMJ or FMK.
The FMJ4B is what was on most of the B's and many other Allis Chalmers tractors. I can't say for sure that the FMX was actually put on an Allis Chalmers tractor from the factory. The FMX was apparently one of the last magnetos made (and therefore the most updated) before everything switched to electronic ignition. But the FMX mag should work.
I'm no expert but this is what I have gathered from asking similar questions about my mag. Hope this helps some.
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