This is a pretty rare model of VW Bus. It has extra windows along the top and the front windscreen is split (all pre-1967 buses had split windscreens). So, it really is just a reference to the number of windows in the vehicle. 8 on top, 10 on the sides, 1 in the back, and 2 in the front = 21. This is a top of the line bus and its rare to find these today.
UPDATE: A reader on Flickr corrected me that this is a 23-window bus. The rear window is not one piece, but three pieces. You just can't see it in this picture.
