[wkooiman]

Looks like pieces of a gear to me. You said you checked the distributor gear, and a piece was missing from the collar. What collar?

Maybe you lost a chunk from the distributor gear on the camshaft, and a piece took out a chunk from the collar.

Also check your valve springs and retainers. I ran titanium retainers for a while, and the springs dug into the retainers. I didn't find chunks in the pan, but I could see how that might happen.