Did that spare starter happen to come from a later model engine- - - -55 to 62 models have 12 volt starters and require a different flywheel. The quick way to check is the diameter of the armature shaft- - - -a 12V starter is about 1/2" in diameter, and a 6V will be between 9/16" and 5/8". Both drive gears have the same number of teeth (9, I think), but the tooth angle is different.
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!