Heres the deal, many BUT NOT ALL typical old circa 40's to 60's tractor (NOT talkin high performance or after market or elec ignition mind you) Kettering Ignition points and condensor type ignition coils designed for operation at 12 volts nominal that will produce a spark when battery voltage is lowered to maybe 10 volts or raised when charging to 14
ARE AROUND 2.5 TO UNDER 4 OHMS (3 Typical)
Many typical old tractor coils designed for use at 6 volts nominal (and all the above but half the values mentioned)
ARE AROUND 1.25 TO UNDER 2 OHMS (1.5 typical)
Many 6 or 12 volt tractors ALL USED THE SAME COIL its just that if used on a 12 volt tractor they added a series voltage dropping (12 to 6) Ballast Resistor which droped 6 volts leaving 6 on a 6 volt designed coil.
Now before anyone has a calf and can find a coil somewhere different from above note: THESE ARE TYPICAL VALUES to which like everything else there are indeed exceptions!!!!!!!!!