M1 Carbine Receiver and Bolt Dimples

[NavyEngineer]

I pulled a few of my carbines out of the safe today to inspect and oil them, when I noticed a small dimple on the receiver and bolt of my IBM M1 Carbine. I did a search, but didn't see a previous thread on this. My guess is that the dimples were used to verify the hardness of the metal - does anyone know if this is the case, or if they signify something else? Thanks in advance.

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