I stand corrected, but I knew it was a jack cover. Old age. For the record, my Grandma had slant 6 Darts and Valiants from the early '60's until her last one in '76. I guess the nut doesn't fall far from the tree.
I am curious, how long did she keep them untill trading in? I figure high mileage in the 70s was 60K and 12 K a year maybe 5 years tops? People didnt commute like today.