I think he's referring to the 6 stamped metal plates that are screwed to the body and cover the holes that were required to accommodate the T-bird tail lights. Surprisingly they are not shown in the 1968 Shelby GT 350/500 Illustrations, Assembly Procedures & Parts List manual.
I think they are a hex head, 1/8" - 3/16" dia by 1/2" - 3/4" long, black oxide sheet metal screw, or something like that.