1) I believe (NO Warranty) "some" Farmalls used a 6 volt coil on a 12 volt tractor but the ballast resistance was in the form of a resistive wire link NOT a discrete bathtub style looking ballast, so if that's what you have and its still wired that way you need a 6 volt coil...
2) In GENERAL if its a 12 volt tractor and there's NO ballast device, you use a full 12 volt rated coil no ballast required
ORRRRRRRRRRRRRR a 6 volt coil PLUS an external series voltage dropping (12 to 6) ballast resistor
3) A coil labeled "12 Volts" or "12 volts NOT for use with ballast" is a full 12 volt rated coil
4) HOWEVER a coil labeled "12 volts for use with ballast" is in reality just what it says and is more like a 6 volt coil
5) Many (not all) 6 volt coils have a LV primary winding resistance of 1.2 to under 2 ohms while many (not all) 12 volt coils have a LV primary resistance of around 2.5 to 3.5 or so ohms
NOTE since this is an old tractor forum I'm talkin typical old "farm" tractor coils NOT many of the wide variety of auto or truck coils as some American made cars n trucks used 6 volt coils and other 12 volt coils with different ranges of ballast resistances to match the different coil resistances etc etc etc ................
I have found full 12 volt or 6 volt coils at NAPA and they cost less then if bought at Deere or IHC etc