I believe it is an International (I) 28 foot (28) 1963
(3) not sure about the "D", 235th trailer produced. You may want to spend some time on this site: Airstream, Inc :: 1960's
Airstream has posted a wealth of knowledge.
Also, welcome to the wonderful world of Airstream ownership. There are a couple of very active groups in GA. One, "The Georgia Unit" owns a campground North of Helen, GA and holds rally's there. The other has just formed and is called the Southeastern Camping Unit and emphasizes camping all over the Southeast. Mostly GA and FL State Parks. Send a PM if you are interested.