A plain old 12 volt coil still only runs on about 6-8 volts. That's what the resistor (external or internal) is for, to drop the voltage to lower the amps through the points so they survive.
Check out an old Motor's Repair Manual, they have a pretty good write up on ignition systems. Quoting from a 1963 truck one:
The ignition coils used with 12-volt systems are specially designed 6-volt coils which operate with a resistor....
I was taught that a 12 volt (external resistor) coil was just the same as a 6 volt; interchangeable.. Seems that's not entirely true according to the motors manual, but not that far off either.
Grigg
