Acura Forums - Acura Enthusiasts Forum - MDX 2007 What Parts? Where is the Location of This Parts?

Car: ACURE MDX 2007
I am doing the front engine head repair-changing failed exhaust valves of # 4 cylinder.
When I removed this engine head away from engine body, two identical parts were found on the surface of piston (cannot remember which one). I thought they were just pins for assembly.
The dimension of the part: length: 10 mm, diameter: 6 mm. It is a cylinder shape but with a slot (opening) along the axle direction.
Please refer to two pictures: the parts on a white paper and a hand helds this part.
My questions:

What is it?
Where does it come from? So I can install them back.

Thank you all for your reply
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