William, with all due respect, I believe you're confusing cord material with belt material. Rayon, Nylon and Polyester were all CORD materials, never BELT materials. Conversely, fiberglass and steel are always BELT materials and never cord material. There is a major difference: The cord makes up the structure of the tire and runs at an angle or "bias" to the lateral centerline of the tire. Cord is essential to the tire and it is worthless without it. Belts are an "add on" on top of the bias ply cord and run laterally around the tire. They are not essential to the tire but do, of course, offer many advantages.
I have numerous technical articles from the 60's in my library when this was all cutting edge. From these articles, bias belted tires (which were ALL fiberglass belts with various cord material including nylon and polyester) were introduced to the general public in mid 1967. I have found no evidence that Chevrolet offered them during the 1967 model year, though, which makes sense to me.
PY4 and PY5 (sorry, I said PW4 and PW5 in a previous post which are, in fact, the non-belted red and white versions) white and red stripe, respectively, are fiberglass belted tires, likely with polyester cord, made optional sometime in 1968. My April 22, 1968 Dealer Price Sheet shows them. They are also listed for all of 69 and 1970 per Rick.
Dave