They likely are (should I say possibly ?). There are several vendors that carry them as reproductions, GM fiberglass had to be exceedingly rare back in the day. I've known two cars that had them. one in '71, the other in the 90's, whcih was sold to a large Camaro parts vendor for mucho store credit (thousands) in the 90's, local car. The one in '71 was on a Z28 crossram equipped street car, Knoxville, TN.
Regards,
Steve