Summerkid, 2 T's
Making some assumptions.
Airstream seems to have bought runs of items. 3500+ Armstrong AC units come to mind. They were still installing them for a few years after the Armstrong Company quit making them.
I would think the bumper for a number of years would be the same across the line. As long as the design of the coach and the frame stayed the same why change it? I would be willing to bet in the 50's it was a automobile or truck bumper that was already in production that Airstream used without holes drilled in it. Just called one of the big 3 and said I need 500 bumpers for a 53 Chevy pickup.
One of the sites I frequent and the one the picture of the 59 caravaner came off of it
http://www.vintageairstream.com/archives/index.html
He has most of the models made pictured up into the late 70's. Helps on the research when you are looking at a picture on the web and the description seems a bit hinkey.
The biggest issue with using the site as a bible of sorts is that things like bumpers get damaged and then they are replaced without concern for originality and that makes it hard to know.
On the ones I see of this vintage the bumper in the pictures of this thread look to be OEM, others made from flat steel I would bet are replacements that were made for the owners one off, or by the factory as a replacement part. There is just no way to "Know".