EDIT: Seems like this is more common than I thought. The deal here is that the Adder in the pics is the variant you find in Story Mode (the one near of Vangelico). I don't know if a past patch screwed something but there's a second variant without that black thing in the back. This isn't a wing like the spoilers in other cars, I just find this weird because I owned a few Adders before and no-one had that. Sorry if this confuses you, but there's no way to edit the title of the thread.
(Proof this is online)
I don't know if this is old information but with all this millionaire stuff maybe some of you want this variant (because it's rare and stuff). I'm aware the Adder isn't the favorite in the forums but seems like purchasable cars with a variant (like the painted top Voltic) can be obtained.