Tylinol said

I may end up needing to get a borescope to verify this - it's hard to see in there, but what it *looks* like is that there was block casting on the path between the oil sender hole and the main bearing cap hole. My hammering punctured this casting, and I can see what looks like a little bit of space between where the casting used to be and the plug (or whatever it is).

If that's the case, I'm guessing this is the original "plug", since there would be no way to remove it without puncturing that casting.

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