I've confirmed with the metallurgist at METL that the "contamination" between the barrel and top rib extension wedge is manganese sulfide and metal oxides ie. corrosion and "burned" (over-heated) surface steel and filler metal. "Burned" doesn't mean "melted" ie the surface of a cook pan left on the stove is "burned" and discolored.
Purposefully not understanding semantics in order to argue is of no help, but we are appropriately impressed.

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