I looked in my Owners Manual "A guide to trailering in your 1967
airstream". It refers to the model & serial number plate on page 15 of that manual. You are correct, it was located next to the entrance door. My manual does not refer to any other location on the trailer.
I don't know how much our trailers cost new. I purchased mine 10 years later and paid $5500 for it. I took it to a couple of dealers for different things + advise. I was told that I had paid $2,500 too much for it!
I've had most of the problems that are discussed here on the forum. And, I've done most of the repair work myself. It's still on the road, and will last me the remainder of my lifetime. Just put new tires on + new refrigerator this summer. I've have had a lot of enjoyment for the money I've spent!