..1970's were flat
..late 1971 were ridged and 1972 were ridged. Mopar put the ridge in late 1971 as a production change because it added strength to such a large flat panel. People were closing the trunk lids by pushing them down on the center, rear part of the lid and some were buckling, denting.
BTW Scott my Duster was built on July 9th too! #452494