Willys used to offer hardtops by a couple different manufacturers, and one of the main ones was a company by the name of Koenig, who also made winches back in the day... might try to see if there are any painted-over ID tags on the top anywhere.
http://www.film.queensu.ca/cj3b/Hardtops/Koenig.html
also, here's a shot of the ID tag you're looking for:
Like Marc said, there were a lot of options for Jeep stuff back when... but not all that many hardtops and the early Jeeps are fairly well-documented-- not to Mopar standards, but what is?
You should be able to get a good, authoritative answer from the old Jeep guys out there without much trouble.