Not sure if you are still tryign to find anything out about that but I'd say with a high degree of confidence that the official GM hard tonneu covers for the SUT were made by Steffens Enterprises aka Fold-a-Cover. I have a Fold-a-Cover G4 on my GMC pickup and the side rails, panels, and other bits are pretty much identical to the factory GM ones for the H2 SUT's.
They have spare parts for their main covers available on their website but I'm not sure if any are specifically for the H2 SUT covers.
I will say this though. They are not the strongest covers around but they are not terrible. Eventually the rubber covering over the long piano hinge does start breaking down some and it will start leaking. I have not tried replacing it on my pickup but I think the method is you have to drill all the rivets out of the hinge, put the new rubber strip on then rivet the hinge back onto the panels.