Your statement "There wasn't an external resistor but the coil had resistance in the primary windings so it was the same as the coils you buy now that say no external resistor required. The resistance is internal." is correct.
However, comparing HEI system to the old points system seems to me like apples and oranges. Appears to me that any coil that will work, as designed, on a 12 volt tractor without extra external added resistance is a "true" 12 volt coil for that circuit.
Contrary to what many guys seem to think, there are no separate resistors in any coil from the points ignition era. A coil is a transformer with one end of primary and one end of secondary tied together. The designer controls the voltage or current capacity by the ratio of primary to secondary turns, the number of turns and the type of wire used.
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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