Hate to bring up another issue with the manual tonneau but just discovered this on mine. Bolts are starting to rust on the underside on 3 out of 4 panels. Live in the PNW so it rains a lot and is damp all the time.
Looks like the end of a steel mandrel from the rivet. Likely the rivet may be aluminum or coated, but the mandrel may have been standard steel? The others don't look as bad and the one with no mandrel looks fine. I'd spray some anti-corrosion spray into the openings and not really worry about it.