When GM went to 12v in 1955-56 they changed the tooth count on the flywheel from 139 to 168. The flywheel was the same size. Both starters had 9 teeth but were not the same size.
"You can also measure the starter pinion gear which maybe easier, the 168 teeth flywheel starter pinion meaures 1" and the 139 tooth flywheel starter pinion meaaures 1 1/8" so a bit larger pinion on the 139 teeth flywheel. FYI both starter pinion gears are 9 teeth, the difference is pitch of the pinion teeth."
I think this is it! Bean counters & tooth counters be dammed!
