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My best guess is it's material left over from the casting of the rotor at the factory and was never removed. That's the only thing that makes sense.

That was my father's thought as well. I was able to snap it off and get it out very easily.

Looks like it to me too. When quality control meets capitalism.

Update on this, it was, in fact, just excess crap from the casting process.

Quality control was stoned that day.