Probably a gunsmith marked the bolts that way & put matching ones somewhere on the actions to ensure that they could be identified & kept as matched units.
Just an engineer's take on it - no specific rifle experience to back up my ideas.
I did baulk at a mates suggestion, maybe done to raise a surface, I was more in the ballpark of proof maybe, but hardness testing didn't cross my mind .. makes a certain amount of sense though.
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