[Brian]

The above theory is correct...however, I ran my 35 V8 for years on 6 volt, using some sort of unmarked coil brought in from America. It has a black body and a gold sort of coloured top. Years later, when I converted that vehicle to 12 jolts, I kept the same coil, same configuration, no ballast resister [just as I used it with 6 volt], and it has run flawlessly like that for years.
Some years ago I imported a 1951 8N tractor. It too was 6 volt with the same sort of coil [black body/ gold top] When the 6 volt battery expired, I replaced it with a 12 volt battery, again leaving the coil unchanged. It too has proven trouble free.
Now, I don't know the coil resistance, however, I understand these are 6 volt coils. They run perfectly fine on 6 or 12 volts.