This is the round metal threaded disc that attaches the D0ZZ-6523258-A bushing to the door glass brackets. About 1-3/4" in diameter, two each per door glass on a 1970. 1971-73 Mercury Cougar and Ford Mustang use 3 of these for retaining the door channel bracket.
Ford part number 384289-S.
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Do you have questions about Glue-In vs Bolt-In glass? How about how to properly install Glue-In glass in your 1969 Cougar or Mustang? Check out our video below for some great glassy information!