To add to what azflycaster posted, another element that can cause confusion is the number of coaches that had "custom" modifications to suit the customers order when new. While it was becoming less common in the 1960s, Airstream would still make "custom" changes to a coach to a customers' order. I know that my '64 Overlander was special ordered by its original owners, and it had several modifications to the standards illustrated in the brochure; and I am aware of another '64 Sovereign
that had additional windows beyond the standard that were ordered by the original purchaser. It is possible, that the coach in question may have had some custom touches from the factory -- or they may have been added through later updates.
Good luck with your investigation!