Oh, I did find something... in April of 1983, the part number 384472 was superseded to 22519181. But the latest GM part number was discontinued 10 years ago in 2009.
Maybe that one was rubber?? But the old parts books shows that it just fit 64 330, 65-74 V8, and 75 260, A and B 350, and 455.
Found some pics too of these early ones. A couple of them are cork too, and appear to have metal insert. First pic has some bends in it that you wouldn't get with just cork. Last one may be rubberized??? Not sure.