Anyone seen this on back of Receiver?

[eb in oregon]

I thought maybe since it was a subcontracted part, when SG got a hold of it, maybe they tested it to make sure their subcontractors were hardening it properly. It is awfully big for a hardness test though

They would test it up at the front of the receiver around the chamber area, the back heel means zip. And again that is not a dimple from a hardness tester. To big, way to deep. And not a ball peen. No way. If anything I've ever seen it looks like a 5/32 ball mill. To perfect for a defect and no ball peen in the world can peen that deep or that small. Nor a punch.