Don,
I think the six volt starters matched up with a 139 tooth flywheel and the 12 volt starters with 168 tooth flywheel. Lots of people ran the 6 volt starters at 12 volts, or had them rebuilt for 12 volt. The two starters mount up the same, but they don't tango when matched with the wrong ring gear. The flywheels can be swapped back and forth just like the starters. Makes for some loud combinations. Start by counting teeth on the ring gear of your flywheel. Then count the teeth on your starter. I've got a 168 tooth on my 235, and a 139 in the attic. Lots of these old flywheels have bad ring gears, look for damage.
