Hi 63Air and all,
First, a little history...
Airstream used the Hehr Hall-Mark 12 windows from 1961-1965. To my knowledge, they have never appeared on any other brand of trailer. In 1966
, when Airstream stopped using the Hehr windows, Hehr discontinued the Hall-Mark 12.
While these windows were still in production, they came standard with a brittle plastic glazing strip. Sometime after the windows were discontinued, so was the brittle plastic strip. Hehr still needed to provide support for the windows, but they did so by producing a vinyl rubber glazing strip to replace the brittle plastic. I'm guessing this was a cost savings measure, but it could have been a practical concern about shipping long plastic strips when they could just ship rolls of cut-to-length rubber in bulk.
To this day, that vinyl rubber substitute part is still made with the original material on the original tooling. To my knowledge, there is no alternative part available at this time...and there hasn't been an alternative for many, many years.
The problem you are having with shrinkage is standard with all vinyl rubber extrusions. The blue vinyl rub rail on many Airstreams shrinks the same way. And so do old door and vent gaskets and so does the original glass seal for the older 1952-58 windows. Vinyl rubber is an "old fashioned" type of rubber and while it is highly resistant to abrasion, shrinkage is the down side.
When installing, some of the shrinkage can be reduced by doing as some have advised: "bunch," don't "stretch." But even those who have been careful not to stretch it will eventually see some gaps in the corners.
What we are doing about it...
We have made a commitment to recreating the obsolete gaskets for all post-War Airstreams. We're almost finished. We're tweaking the tooling for the few remaining 1947-49 trailers now. And we have the 1950-51 gasket prototype en route to us as of last week.
Of all the extrusions we sell for 1947 to 1965
windows, this is the only one we did not develop the tooling for. It is the only one made out of vinyl. All the others are done on tooling we've developed and they are made out of silicone rubber. Silicone is much, much less prone to shrinkage. Just this month we have finished designing our own silicone version of this bead. It is now at the toolmaker. When tooling and testing are finished, we will have it for sale. It won't be as "original," but it will be more stable.
I hope this post doesn't come across as too commercial. (If it does, Andy, please don't hestitate to delete it). I just wanted to add some background for the discussion.