69 barracuda fastback back glass gasket

Majority of the gaskets for 68 and later are 1 piece self locking. One lip folds inside the other. I have seen only 1 rear window gasket on a 68 notchback that did have the separate round rubber locking cord in it. I really can't imagine it being OEM but there were parts shortages in 68. Did they toss a vendor sample in the one build? Who knows. We can't really know what the aftermarket might have produced decades ago either. I know that 2 piece type gasket is a lot more flexible/workable for glass R&R.
If you did cut up a original self locking old hardened rubber gasket to get the glass out without breaking it... I understand. That is the safe way to do it. So buy a new gasket.
Most of us will install the glass from bottom up, gravity being in our favor but... getting the top of this gasket to roll in and lay against the headliner like it should has been a problem in some cases seen here. I submit that this is why the factory service manual states install glass from top down. Good luck with yours