MTFCA Forum

The Allen Bradley Co. made this for all cars with a generator with a third brush just like the Model T Ford
has anyone ever seen this? Bob

It looks like a carbon pile resistance to shunt the generator output to ground, in a lesser or greater amount. Assuming this is correct, as the knob is screwed in, the carbon discs are pressed closer together, lowering the resistance, which shunts more of the generator current to ground. The generator still puts out its normal charging current, but less of it gets to the battery, instead being converted into heat. It's the same principle as turning on the headlights to prevent the battery over charging, except this device allows a variable adjustment.
Allen Bradley & Co. was a resistor manufacturer going back to the early days of radio and their rheostats went under the name of "Bradleystat".