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It's hard to tell from a photo, but it looks as though it's raised, in which case it's probably been there since it came out of the mold. I wonder if there was a crack in the mold that allowed a little molten aluminum to fill the crack in the mold.
Absolutely from the casting process. These were cast in compacted sand molds. Sand molds can crack and the casting will fill the crack. The tell is the plus metal in the “crack.”
Crack or casting flaw
You will have a hard time finding one without that cast
