I actually came across a version of this trailer for sale on Craigslist in July of this year. It was 28 ft long and painted just like the picture on this brochure, but it said "Airstream" on the endcap, not Argosy. It threw me for a loop, because it looked like a weird combo of both trailers. It was missing the panoramic windows of the Argosy line, but had the steel endcaps & paint like the Argosy... Yet it said Airstream... Gave me a headache!
I printed out a copy of the ad, just for fun. The seller said it was a 1980 Airstream and was asking $7500 for it. They said it was in good condition and had 4 pictures of the outside, but unfortunately none of the inside.
The ad is long gone from Craigslist, but I keep hoping to see the new owners here on the Forums!
My guess is that in late 1979 they had already built some of the trailers before they decided they weren't going to do another Argosy model year. They then put the Airstream name on the endcaps & sold them. Makes sense - why waste a perfectly good trailer, or try to strip & clearcoat it to match the other Airstreams? Much easier to just sell it "as is".