I've never seen that before on a factory part.
Since it's a loose part and can't be verified as a factory alteration, about all you can assume is it's something someone did after-market for unknown reasons.
Its from a complete... lightly used LE6933 upper. This guy has a few of them on GB Auction # 349994650. From all the pics he has it looks new to me, but as you suggest it might be some kind of mod, but for what?
With my small sample size of my previous pre ban AR and currently two 6920, two 6721, and one USGI M16A2 Colt upper half on Match Target HBAR A2 fixed stock lower half, and 42 years being a Colt owner I had never seen that marking on a bolt carrier.
My best guess would be that it is some kind of Rockwell hardness testing mark, but those markings usually have rough edges, maybe someone cleaned it up by drilling the marking to smooth the edges to avoid any start of possible cracks.
That dimple should not have any affect on the function of your carrier, I've seen competition lightweight carriers with a lot of large holes like a swiss cheese and they work fine.
I've seen a lot of Rockwell hardness testing but at my company we mostly do it on parts excess material then we trim off the material with the hardness testing mark, maybe the Colt factory made a mistake on the location of the testing.
I'm an aircraft senior manufacturing engineer with over 20 years experience from major and small aircraft companies.
Is it all the way through? It appears to have a nice chamfer and the finish is the same as the rest of the carrier so it probably came from the company like that. I too have never seen such a divit from a hardness test. It could just be the result of a test of the barstock material used to make the carrier.
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