Pull the cover, take out plugs and always remember I said to abandon the starter and buy the proper one and you said it's a joy to fix it.
Both 6V and 12V have 9 teeth but different pitch. Hard to tell one from the other, usually go by the armature shaft diameter if no other way. In your case I don't know if I would trust that or not. Now you say you notice a difference in pinions. 6V pinion gear is slightly larger than 12V supposedly. 6V flywheel = 139 teeth.
I believe "slightly larger" pinion means OD. But shaft diameter is the approved method. You need calipers for that stuff not a carpenters tape.
