ANGLE DRIVE SURPRISE
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Porosity in the casting. Could be caused by dirt or gas. Likely dirt, as I'm nearly positive this is a sand cast part and not die cast. Can be caused by the sand rubbing when the mold closed, or it wasn't blown off well before closing. I'm lead tool designer at a prototype/limited production sand cast factory, I see this kind of thing all the time. When the metal is poured, it will wash any loose sand no longer held by the binding agent to a swirl area and settle. The area it settles will be the same place most times, rarely will the sand end up anywhere else. This is a process error from the foundry that made the pour.