No its not a piston ring. This is basically a very thin ring shaped strip of metal inserted into the inner top portion of the first groove of the piston during its manufacture. (The compression ring is assembled below it). I cant seem to find any related pictures or links of it. here's a rough drawing.
Is this for an ICE, a compressor, liquid pump, hydraulic cylinder, etc... ???
What materials are the parts made out of?
Does the shim seal on the top of the compression ring?
Does the shim wear on the OD where it contacts the cylinder?
What kind of pressure and temperature do the parts see in operation?
I've never heard of such a thing, and I've seen a lot of pistons. As others have mentioned, it resembles a component of a 3-piece oil control ring, but those aren't a pre-installed part of the piston, and don't protrude farther into the skirt than any other ring.