I thought the Berry plastic decklids had some edge detail/bevel along the bottom of the decklid, carrying over onto the end caps. Am I mistaken, or was that a different company?
Don't believe Berry was making plastic trunk kits back then and don't recall any edge oddity that I can recall, though they did use the term "Plasti-glass" in their advertisements of the time.
The quickest identification feature was the lack of provision for the Mustang or Shelby trunk lock. This detail can be seen in ads for the trunk lids in the later 60's
If the one in the ad is indeed from the car mentioned above it's fiberglass like the one installed on our car not sure where the "Plasti" comes into the process
Have pictures of five 65-66's a number of them from the rear with this style of trunk lid.
On the old site there was a fairly long thread discussing different peoples belief on who was the first to design and come up with the design. Think there is still a web site up with one persons story.