Hi Alex
after reading your post I also went on a quest to find out if you are right and yes you are I not only found photo evidence like your picture but I also looked at my CMP Fat again and yes I believe who ever converted the truck used the roof to fill in the back and the vents still work
when researching I also notice that although the Beatle back looked the same their were supple difference's some had the rear seats facing the front so crew sat over the rear wheels and I think the later version had a bench seat for the crew which was located behind the front seats facing towards the rear. also the back of the body doors are two big ones and on others its 3 little ones different sizes. I would like to know the reason for this and is this a modification difference between dates