There is a different drive assembly for 6V and 12V starters. A 6V has straight drive lugs on the armature shaft and a shaft diameter of around 5/8". A 12V starter has spiral drive lugs and a shaft that's close to 1/2". Some 12V conversions retain the 6V starter- - - -which really spins the engine over fast. It doesn't hurt the starter as long as you don't crank for a long time. 6V Delco starters and generators have a black painted ID tag- - - -12V tags are red.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!