Found my dads original 1971 duster!

Bennoel 10 said

I believe the reference is between Dusters (70 and later) and Dart sport, Demon bodies (71 and later). The 67 to 76 Dart is much different.

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Based on the problems he is having, I bet he has Dart doors, not Dart Sport doors. And the "builder" tried to make Duster/Dart Sport glass work in a door that was supposed to have wing windows.