The more I think about it, the more I recall the same as you, on all the '60-'66 A-bodies: once the metallized mylar flakes off, the base plastic lockstrip has a smooth surface without the ridges.
This type of mylar coating was used for a number of years on a whole variety of Chrysler vehicles. I was on several of the windows of my '86 Dodge Maxi-wagon as well as the liftgate window on my '79 Aspen.
I believe the mylar was discontinued, probably as a cost save in the early 1990s.
