Here's a question- A friend of mine has, among his collection, a 57
Westphalia, with what I would swear to be a factory installed removable
roof section. It is basically the same size as the roof of a single cab.
It has a factory looking seal, is fastened by the exact same fasteners
on the "pop-top" metal flap, and the fresh air vent matches up
perfectly. I was just wondering if anyone else had ever heard of such a
thing. By the way, he also drives a '63 single cab, his wife a '68
kombi, and he has a '61 Westphalia as well. 
Thanks,
Richard