My manual says the 12 volt OEM coil has the resistor built into the coil. The resistor in this circuit is not going to do a lot to reduce the voltage but is there to reduce the current used by the circuit. The current is what produces the heat and destroys the components. Here is what it says:
1. The ignition coil steps up battery voltage high enough to jump the gap on the spark plugs. The coil is mounted on the fan housing adjacent to and above the distributor.
2. Inspect the coil wires and insulating tower for damage, splits, cracks, and carbon tracks.
3. Check the routing of the wires to the coil. If the wires to terminals 1 and 15 are reversed, the engine may run, but will misfire under certain conditions.
4. Using a voltmeter, turn the ignition on, engine off (do not operate the starter) check for positive voltage at terminal 15. On 6 volt models, at least 4.8 volts should be reaching terminal 15; on 12 volt models, a minimum of 9.6 volts must be available. If not, trace and repair the wiring.
5. If sufficient voltage is reaching terminal 15 on the coil, disconnect the high voltage wire (terminal 4) at the distributor cap end and check for a spark as described in section six.
6. If there is a strong spark, check the ignition points and condenser and replace as necessary.
7. If the points and condenser are OK and sufficient voltage is present at terminal 15, replace the ignition coil. Caution: Use the correct coil for the application. Coils used on 12 volt models have an internal resistor to reduce power going to the windings. Models with 6 volt systems do not have a built in resistor. Use of a 6 volt coil on a 12 volt system will result in burned points; a 12 volt coil on a 6 volt model will not produce enough voltage.
I know you did not need all that information but I included it directly in the test section from the manual. According to this I suspect the coil does not have the resistor and the current passing through it could eventually cause failure. Are the points burned, that would be an indication of the missing resistor.