Thank you for your reply.
I apologize for the rough sketches, but the shafts seems to be identical, except for the splines (and the prop nut attachment).
Further research would seem to indicate that the switch to a 23-spline was 1958 for Johnson…I can buy a 23-spline dog, but will it properly engage the ’57 gears? Am I falling down a rabbit hole that I ought not go down?
I think your assessment is correct. I also think your question about the dog fitting the gears is valid. But I don’t know the answer for sure. Yes there was a change in the outside diameter of the dog back around that time. Now the question becomes is it bigger or smaller? That, I don’t know without having the parts in front of me