6 volt or 12 volt coil?

I have a 59 CJ5 with the 134 engine. It has a standard 12 volt electrical system. I went to NAPA for a new coil and their book shows a 6 volt coil. They sold me a 12 volt coil, but it got me wondering which it should be, 6 or 12. Is it really a 6v coil, or are the NAPA books wrong? If 6 v is standard, does it hurt to use a 12 v coil?

On a related note, should this system have a ballast resistor? I would think that since its a stock 12 volt system it would not have one. There isn't one on my Jeep, at least not in the engine compartment. I also have not found one listed for that year/model in the on-line Jeep catalogs. I I have read about other vehicles that are 12 volt that use a 6 volt coil with a ballast resistor to aid with spark at start-up. Were these early Jeep 12v systems like that?

Thanks, Mike

 

Its about the points. Any more than 9 volts to the point will fry them. manufactures solve this with ballast resistors or resistor wires. 6 volt coils are for 6 volt battery systems