Was it really because American men have bigger arses?
I thought that the aim of the hook vent was to strengthen the vent. It's only a thought and someone will probably correct me.
Yes - strengthens the seam - takes the pressure of the stitching. And, when you think about it, lapped seams lend themselves to it, no?
I don't know the reason for the ol' hook. But it's a detail I like. And got on a more recent Andover coat.
I thought, given the seam in the back (like in the ol' Brooks Makers tweed sacks), it was a necessary thing to keep it all together, but I could be wrong.