There is no set situation that can answer all this except a Chrysler Bulletin on or if the assembly was done inside the plant or outside the plant and when all this might of changed if the assembly changed. Many a 1971 E Body had things added to the vehicle when it was sold and a stripe (billboard) was one someone wanted put on or wanted taken off. The maker light assembly was to have the markings for either black or white billboards, did all Cudas get these marker light assemblies put on the car from the factory, maybe not.

So the poster needs to look at his marker light assembly and then ask previous owners about if it had billboards and ask them if they know any other owners before them and if any of them knows the original Dealer that sold the cars. He can ask each person if it had billboards and if the dealership remembers.