js27:
I finally got around to looking at my 1971 Barracuda manual convertible top; it did not have those black square plastic things. Reflections7's picture shows the two unique-to-manual-top hold down brackets (the metal brackets with the white plastic end), which my car has, but I have never seen those black plastic pieces on my car.
Stwheels, you say that every car you have had, manual or power top, had these black pieces. Were they attached to the folding top frame? Please, help js27 and I out so we know what parts we are missing. Thanks.