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If the coil states "Use with external resistor" it definitely needs to have a resistor used with it to reduce current draw through the points to a manageable amperage when used with a 12 volt system. As far as your other tractor, you may have just been lucky, or have not put enough hours on it to burn the points. The make of the actual distributor should not matter. One other possibility is that the person converting it to 12 volts used a length of resistance wire in the harness from the ignition switch to the coil. Some cars and trucks in the late 50's up into the 70's used the length of resistance wire to serve the same function as a separate ballast resistor. With your NAPA coil, you will need a ballast resistor. There are two types. One type is a fixed resistance of somewhere around 1.5 ohms. This ballast resistor is designed to be used with a start bypass circuit from the starter solenoid "R" terminal to the coil side of the ballast resistor to improve spark voltage available for starting. The other type of ballast resistor has a positive temperature coefficient, so it will have about .60 ohms resistance when cold, as in when you first turn the ignition switch on to crank the tractor, and the resistance will rise to about 1.5 ohms, or so after the tractor has been running for a minute or two. This second type of ballast resistor gives good spark voltage for starting, but, yet, the resistance rises soon after starting to allow good point life. No starting bypass circuit is needed with this second type of ballast resistor. A knowlegeable parts counterman should be able to find the resistor(s) that are listed as compatable with your particular coil. I generally use Standard Ignition (brand name) parts, and their catalog lists the proper resistors to use with their universal coils, and I'm sure NAPA would do the same.
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