[Rockin Robbins]

Well, you can tell by the conning tower that the one with more vent holes was a later war boat. I imagine that the extra holes made submerging quicker to avoid those air attacks that people keep denying happened so much around here.

The faster you can move air out of the outer hull area, the quicker you will submerge. It makes sense that they would add more.

But that's only a logical approach and logic doesn't work with history. So it's time to call for Davey!!!!!!!