I am no expert, a novice on this topic, actually. But I do know over the years several different types of "frames" were made. Tubular types in the 50's (maybe 40's...) Almost box frames later, then channel type metal for frames in the 60's. We have vintage stuff, so I can't comment on newer Airstreams. I think the style and design was also a bit different on models that were dual axels, etc.
Interesting email you received.... can't imagine he has been there long.
Mrs. NorCal Bambi (traveling in S Tardis)