ESB 21 Back Proof
As "The Empire Strikes Back" is the film where Chewbacca shines the most, ESB items with Chewie are a favorite at the 'seum. This here is a preproduction proof of the first ESB Card for Chewie, so it is one of the first possible items with this character in this film.
Before a card is actually printed and fitted with card and bubble, there are printing proofs made at the factory. There are anywhere from 4 to 50 versions of any given proof. There are several kind of proofs, but this one here with the squared bottom corners, is the most common. I figure there are about 10 of this particular card out there, but I could be totally wrong.