Here is a 1945 American Bosch catalog for farm equipment magnetos. This may help determine which magneto will fir a particular tractor or farm implement.
The MJA is one of the hottest robust,reliable and just plain bulletproof magnetos ever built in our opinion. They just plain work even when all is not well.
The MJB is also a very hot and robust magneto. it is very similar to the MJA with a slightly smaller rotor and coil with a bronze sleeve rotor shaft bearing in liew of a ball bearing.
The MJC is also a very hot and reliable mag. The engineers probably could have done a better job designing the rotor drive gear mechanism but they seemed to hold up well. Some shops would fasten the diecast rotor drive gear to the armature shaft with a screw.
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