Marks on breaker

Any idea what could cause these marks on this main breaker? Is it significant?

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It’s hard to tell from the photo what marks are you referring to?

RM- i’ll presume the arcing
unless present at time of occurrence…anybody’s guess

I agree with Barry. It looks like arcing. Strange that there is no evidence of arcing on the screws. At least from what I can see in the OP.

Arcing may not be it, since it is on plastic. It might be a black antioxidant paste, even though the paste on the wires is gray. Thanks.

There are normally paper labels covering that area which hide the factory adjustment screws. And no, those screws are not to be turned by anyone.