1963 year was the first of several different anodized skins. The source changed from several different suppliers which is why except for the end caps each trailer was made from the same roll as colors actually varied quite a bit from roll to roll.
Not noticeable until you saw new Avions sitting in rows and then you could easily see the color differences (but only people would could see differences in the silver color) some people could see no difference but women could tell a great difference (something about men having different color rods in their eyes).

The 1978-79 models had the lightest shades of Aluminum but were more sensitive to change due to less anodizing so they had to be carefully attended to with wax to keep that brightness which is why the earlier years were rather dark because they had longer and better anodization (opinions on color and harness vary). Anodization comes in Colors as you can remember the anodized Aluminum Drinking glasses that everyone used to have for summer time that came in different colors (anodizing) along with the pitcher.

1989-90 models are Electro-Coated (anodized by Electricity rather than an electric charged chemical bath) Generally you see this called E-coated (for Electro-Coated Anodizing).

Also because of licensing varying from state to state many of the units are titled with the year being earlier or later than the model year.
My 1964 titled Avion was a 1965 model but because it was made in 1964 the state of Minnesota licensed it as a Mobile Home manufactured in 1964. State of Wisconsin would have done the same thing back then.
So that is why you see some things and models appearing off by a year as they are not licensed as cars by model years in many states.

June was the first manufactured dates for most model years but they were still running the current years models when the new models were being made for the South Bend Trailer Show that used to be held over the 4th of July week at Notre Dame. Avion always had 2-4 models there and they had to be manufactured in June to make the show. The Avion Dealer in Elkhart always got those show models from the show. I would go there the week after the show to see the new models (basically fabrics).