My 71 R/T Challenger came with the H5XX black/white cloth insert seats.
Part of the package was the carpeted panels on the bottom of the door panels similar to what is on the 1970 SE optioned Challengers.
I was cleaning the door panels and removed the carpeted panels to re-dye them and found production info on the back.
I'm assuming these carpeted panels were outsourced and since there weren't a whole lot of these used in 1971 Challengers they probably sat awhile.
My Challenger's SPD date was June 16th
One carpet panel was dated March 22, 1971
The other was April 20th 1971
Probably the person who made or inspected the carpet panels.