Have you every had a valve stem seal job?

The part looks like a ball socket that must be carefully removed with a pencil magnet when a rocker arm is removed. I think they are small enough to go thru one of the oil drain holes in the head.

I don't remember whether or not the ball sockets have a hole in the middle, like yours appears to have. If they do have a hole then it's a ball socket and it may have got there just like I described above.

Edited
I'm almost positive that's a ball socket. And if you know who did the valve stem seal job or other valvetrain work on your car you know who to show that part to.