To confuse matters a little bit (but not much) there's also a difference between the Alclad aluminum used in the 50's, 60's, etc. that isn't used on the more modern trailers of today. Alclad, as I understood it, was a micro-thin layer of pure aluminum that gives you a teeny, tiny bit more shine than the newer trailers are able to buff out to.
I don't know the real world differences between the degree of difference, or when the cut-off date was, but rest assured that as Shari says - any Airstream can be polished up to an impressive degree, given enough patience and the right tools.
Hmm... Valparaiso to Chicago isn't very far. Maybe you'd like to help me finish the polish on mine come spring... it'll be good practice for you!
And welcome to the forums.