Greetings Glyn!
Unless a previous owner has made changes to the tongue, Airstreams
produced up through 1968 had a 2" coupler - - usually a Marvel during
much of the 1960s. When the redesigned coaches were introduced in 1969,
the 2 5/16" ball was part of that package. The biggest concern that I
have had with my '64 Overlander has been finding a 2" ball with a high
enough weight rating to safely tow my trailer - - I have usually found a
properly rated ball by going to my Airstream dealer or a specialty
trailer supply store.
Good luck with your new acquisition!
Kevin