Think when mentioning '68 Shelby, the target assembly plant to compare details would be Metuchen-only. I wouldn't say every single '68 GT350 had the seal, because as soon as I do, someone will show an original without one. The most common attachment of the seal I have seen is with staples, but the big metal clips were used also. As far as screws, I couldn't say for sure as I know that was one method, but not sure if done at Metuchen for any significant amount of time.