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GM parts catalog shows the case no. 1107634...............12V 6 cyl
Chart poted by Kevin in 12/22
We here on stovebolt have been carrying around the statement that 12V shaft is .470 and 6v "is closer to 5/8". Kevin shows 9/16 (.562) for 6V. So we should change that "closer to 5/8"...... to.......... 9/16 .562

55firstseries measures .467 for the 12V shaft. .469 is 15/32 so we should change that .470 to .469 15/32.

Back then, and now, designers/machinist choose fractions, if they have a choice, as a nominal dimension.
Of course there is a .001x or .002x tolerance. Sliding fit.

So armature shafts are 12V= 15/32 (.469) and 6V= 9/16 (.562) as one way to identify a starter. A little more precise than 1/2 and 5/8.