A lot of makes of farm tractors had what is known as a third brush generator, including the Oliver 60 & 70s. The third brush picked energy off of the comutator for amperate flow through the field circut. The field circuit went to ground through the light switch. When the light switch was pushed all the way in the field circuit went through a resistor on the light switch reducing amperatge flow for low charge. When the switch was pulled to the first stop the field circuit went directly to ground increasing the field circuit amperage flow increasing the generator output. When the lights are on the generator output is in the high charge position. The third brush is adjustable for charge rate. The cutout box on the generator closes the circuit to the battery when charging and opens the circuit when not charging.
Not too long ago you could still buy new light switches from John Deere.