I am replying based on my knowledge of my '37. I don't know if the '39 is the same.
My '37 does not have a voltage regulator, only a voltage cutout. This keeps the battery from turning the generator into an electric motor. On my headlight switch, there are 4 positions; 1) Off, 2) remove field resistor (that resistor on the back of the switch), 3) parking lights, 4) headlights.
One of the problems with having no voltage regulator is that you can overcharge your battery. I have been told that if you go on a long road trip you should put your headlights on to help prevent that. Also, to prevent overcharging, the off position of the headlight switch includes that resistor, called the field resistor. Let's say you have an accessory, such as a heater and you are doing stop and go driving. You put the headlight switch in the second position, which removes the field resistor. You can see the result with the ammeter which shows a bit more charging. For the parking light and headlight positions, the field resistor is also removed (bypassed).
Kent
