Where did this come from?

If it is running good without any apparent problems, I would just run it and forget you saw that aluminum chunk.
I would keep it though, so when the engine finally explodes, you can match it up with the other parts during the autopsy.......
Just kidding!
To me it looks like a chunk of the piston or possibly the oil pump.
Was the engine redone at some time, or is it all original?
It might be a chunk left over from the rebuild that worked it's way out of a nook or cranny if it was taken apart or rebuilt in it's past........