[HO455]

This article should help. Basically the difference between 6v coil or a 12 volt coil is what the internal ohm reading of the primary coil windings are. All this confusion started when the automobile went from 6 volts to 12 volts. Instead of making a different coil the manufactures for the most part added resistance to the ignition wire to drop the voltage to the coil. It was cheaper than engineering a new coil and any old coil would work on any car. Plus there was an advantage just down the road.

About the same time engine compression was increasing. The increase in compression lead to more of a voltage drop when cranking the engine over. That voltage drop caused the volt to the coil to drop and in turn the coil putout became weak and the engine was harder to start. To compensate for this the manufactures bypassed the resistor while the starter was cranking so the voltage at the coil was enough to provide strong spark.

https://blog.simonbbc.com/ballast-re...nce-and-coils/