The one I got from a salvage yard was composed of common plywood (maybe 10mm thick) and then a thin veneer was laid on top of that. I used Burl Walnut to match my W111 interior. I imagine they used plywood so they could bend to the curvy shape.
Is the plastic on yours the outer skin or is it the base material? Take a picture of the plastic part. The W111 coupes and cabs took over from the Ponton. I have both btw. While the Ponton c/c was a money is no object build, the W111 had some cost savings. As the model progressed, more things became less handcrafted and used more plastic. Seat hinges are a good example. The Ponton and maybe (?) early 220SE, had a solid chromed steel hinge. No covers over the screws. My '68 280SE has chrome plated brass covers that cover the unfinished hinges. The final W111 used a chromed plastic cover to replace the brass piece.
Based on that, I wouldn't be surprised if plastic became a material used for the tray on the later cars.