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.