There were some coils that actually contained discrete internal resistiors buttttttttt that was more older cars and some had like a ring/junction mark visible on the exterior where the resistor was separated and contained inside but that was more for older autos and I havent seen them on tractors of the 30 to 50 vintage.
What some call "Internaly Ballasted" coils isnt quite correct, the greater resistance is made up of more wire turns or higher resistive wire NOT an internal discrete resistor.
Many farm tractor coils that have say 1.25 to 2+ or so ohms primary (between lil + and - terminals) are for use on 6 volt while if they have 2.4 to 3.5 ohms those are 12 volt coils. Of course they are designed to produce a spark at much reduced cranking voltages and still operate and not overheat at the raised charging voltages
Hope this helps, get out your ohm meter and check them out
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