The "12 volt" coil actually operates on about 9 volts. That's the voltage that's available during cranking with a very cold, or very hot engine due to the load the starter puts on the battery. The resistor you mentioned reduces the voltage to the coil during normal running when the starter isn't operating. There is supposed to be a circuit that bypasses the resistor while cranking to allow full battery voltage to go to the coil, then reduce the voltage when the starter is not running.
Using a 6-volt coil with a 12-volt battery will burn out the points in a matter of minutes, even if the resistor is in the circuit.
Jerry
