Welcome to the group !!!
I have never seen a piston with what looks like a proof mark, other than the Made in USA marks on some of the Tapco pistons.
Personally I would go ahead and fire the weapon, but keep an eye on that area of the gas piston. It shouldn't be a problem as the piston itself should be installed slightly loose on the carrier, and if that amount of slop doesn't beat things to death, I don't think that mark should hurt.
Added a couple of your pictures for those of us who are lazy, and I speak from experience.

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