Had the same problem as 69Boss302. Bought tinted & date-coded door & quarter glass for my 69 GTX from ECS several years ago and installed it shortly thereafter. The new windows were very hard to raise & lower. Finally a couple years later I took a straight edge to the door glass & found both were bowed in the middle by nearly as the thickness of the glass (0.5 cm).

Went back and forth with ECS about them not standing behind their product. Finally they agreed to replace them at a discounted price - $349 for the pair. I haven't installed them yet, but I made sure I put a straight edge to the new ones. They're out by 0.1 cm in several places, but I think they'll be okay.

Didn't like doing business with them the second time, but for the reduced price I did. My problem now is finding a glass shop to do the installation (the one I used before went out of business). I figure ECS's poor quality control & warranty will end up costing me ~$800. If you do buy them from ECS, strongly suggest you make sure at least they're flat across the horizontal plane before much time lapses.