Maybe something from your oil pump?

Check the next filter too...

The filter is part of a thermistatically controlled filter head. When the oil is cold, and the t-stat is closed, oil flows around the closed t-stat, through the filter and back to the engine. When the oil is hot and the t-stat is open, oil flows through the t-stat, the cooler, then the filter and back to the engine.
This cooler has never hooked up to any engine failure.

If I see no other issues, I plan on running it a bit more, then pulling this filter for inspection.

Could it be part of the filter casing? Or a part of the filter?

Just a guess?

wow, this a puzzle, keep us posted. As much as I would like to think it was filter from manufacturing it etc. that is two pieces of FOD right. (sliver holding open the bypass and this. Too much foreign material, I vote tear it apart. Too much stress and money not too.

Well, I found where the small piece of debris that was holdong the check valve open came from. It was a small corner of the pre-luber pump impeller. The mystery is the larger piece.