There are 12v coils that do not have an internal resistor - these need a ballast resistor.
There are 12v coils with internal resistor - these do not have an external ballast resistor.
Just to confuse us, there are also different resistor ratings.
Also, to further confuse someone who has bought a truck converted to 12v, there are "resistor wires" that provide resistance - this might have been used by someone who wanted "neater" wiring.
These 12v systems provide more that 6v to the coil (often 8v-9v).
http://randysrepairshop.net/general-motors-breaker-point-ignition.html
