My PC 500 10.5" Hunter had those marks on the underside on the topstrap and on the cylinder.

I had purchased some of the Hornady 300 grain FTX ammo and shot it all up. (three or four boxes) Some months later I got a phone call from my local gunshop. They had been contacted by Hornady, apparently there was a recall for that particular lot of ammunition. Overpressure.

I wasn't real happy that I had fired overpressure ammo in my PC 500. But it is what it is.
There weren't any overpressure indications when firing that ammo. No loose primers, no pierced primers, no difficulty with extraction.

Hornady sent me hundreds of FTX 500 projectiles at my request instead of replacing the defective ammo. (which had been fired)

S & W Performance Center said as long as the revolver was intact, accurate, and exhibited no lockwork timing issues, it was their belief that no permanent damage had been done.

I believe that the cylinder "grew" enough to touch the underside of the topstrap. The surfaces are bead blasted stainless steel.
It's pretty easy to see where the surfaces touched.
Mine looked EXACTLY like yours.