The plastic plenum dates to the mid-80's. A manager from Ford Parts and Service division brought a prototype by the shop one day to show it to us. I'm thinking it was around 1986. He told us Ford was going to resurrect the part and make it available again. Sometime later it appeared in the price books. They did not last long though. I think they were discontinued within a few years. Probably didn't sell enough of them to make it worthwhile. It was about the same time the first aftermarket reproductions were becoming available. I still have one in one of my cars.
Regards,
Scott