The centre rib bracket is there on the red 6cyl HX Sandman ute in the HX utes brochure (BTW for trivia's sake this ute has chassis number AHX00014A built in 4/76). Just hard to see in the small photo in Dr Terry's book.

I've always understood the holes were drilled in the quarters in the finishing areas of the assembly plants, same place the blackouts and stripes would be added. However it does make sense to have them in the body prior to paint which would mean they were drilled in the body plant.

I'm also wondering if all utes built for stock orders had the tonneau clips fitted, only those as customer ordered Belmont/Holden utes (which as a guess might be 1 in 50, maybe 1 in 100 or higher) where a tonneau was not optioned (option E93) may have not got them. I'd also assume that dealers may have fitted these clips if a car didn't have them but the customer decided they wanted them.

Looking up E93 doesn't give up much info, except that it was also part of the Deluxe (A8J) package in HJ. All the bits for the clips and pins that hold them are in the parts catalogues too, but there is no mention of a kit or anything like that, just the bits.