Mark, you're confused, so am I from what you have just said. The old coil would no doubt have had some marking on it to denote its voltage. Not sure of the history of this vehicle without going back through the threads, but maybe someone "converted" it to 12volt, but had not changed the 6volt headlamps as they did not consider using them, hence why the indicators are 12volt.
I doubt there is a voltage dropper in the system, but just in case there is, why not use a multimeter to check the voltage going to the coil. If 12v, then the answer is to try a 12v coil, but make sure it is a full 12v one and not one intended for ballast resisted systems as in a lot of modern vehicles.
regards, Richard