GMH wouldn't have built them, they would have been done at an exterior cottage industry like most other soft trim and many other parts. So tarp suppliers and also materials put into those tarps may have changed between 11/71 and 12/85 when the W series utes were being manufactured. Note also that E93 Tonneau cover was standard with Kingswood non-Sandman only and optional on XX7/XU3 and Belmont/Holden so like today an aftermarket copy may well have been a lot cheaper than a GMH branded one, so unless a ute is a Kingswood it may be hard to claim any tonneau on one as an original "factory" item.