The second photo is the right way up, so to speak. It has a kind of C section shape and thereby kind of wraps over the pipe. Pardon the multiple posts, doing this on a smartphone is clunky.
To me it looks like the seal that joins the bottom of the waste to the top of the trap. Bottom right in both below.
Edit: PS the C shape you describe is normally due to the thickness of the waste male thread being thinner than the seal itself thus causing an imprint.
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