Mine had that but it was so stiff, cracked and corroded that I threw it out. Also thought it was just some incorporated gasket that was used as a spline. I used regular spline when I installed the screens and been looking for replacement since. It covered the bad cuts around the window frame.
I had the same trim on my 69 Trade Wind. I replaced it with screen spline from the hardware store. It was used to retain the screen mostly. Just trim your screens, and it should look fine. If you do find it, please post it.
This website is not affiliated with or endorsed by the Airstream, Inc. or any of its affiliates. Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.