I recall hearing decades ago that they were based on the Mopar flathead 6 starter, which seemed odd to me at the time.
I think that story may have been de-bunked by now, but it may be that they look alike externally.
I would think the flathead or any pre-56 starter would also be 6V.

Why Chrysler went with that on the stick cars baffles me.
Never liked how slowly mine turned over.