Hy rodC66, welcome, according to my Canadian parts book, the 55A or first series, and the 55B or second series flywheels have different part numbers, of course it doesn't tell me how they are different, just that they have different part numbers. The flywheel ring gears of course will have different part numbers, due to the different pitch between the six and twelve volt ring gears. One thing you might consider, use your six volt starter on twelve volts, it won't hurt the starter as long as you don't have to crank the snot out of it to get it started, hope that helps.
