Well, I would never have noticed the difference between the two fins.
One has the end shaped to allow the extension to slide over.
I can only guess there could be a slight indent in the painted surface where they slide together, but I really can't remember.
I obviously have two different fin styles on my car but they both seemed to fit into the extensions OK. In fact the fin with the flat end aligned the extension piece better when viewed from the rear, probably because it was a snug fit. I'll recheck how it seats on the painted surface next time I get to the car but I think it seemed OK too.