If you look at the rear seat area of a 2-door with the door panel removed you'll see the 4-door opening is there with a panel covering it. It seems that all early B's were built as 4-doors and the B pillar was moved back for 2-doors and the rear door opening covered.

The link below is to my photo album showing the conversion on a wagon, same for a sedan or hardtop:

http://good-times.webshots.com/album/563441250xftUsu