When I purchased my 69 GT500, this part came with it, but I can't identify specifically what it belongs to. It doesn't appear to be a rear valance for my Shelby. It is fiberglass, and has DT 40-1868 in raised emboss on it.
I can confirm it was not used on any year 65-70 Shelby from the factory. Sorry but I don't keep up with all of the after market parts and what they are used on.
1966 1967 Corvette GM 3874799 DT-40-1868 Filler Panel Rear Exhaust Valance Panel
Thanks. U da' man