It is hard to be sure, but it looks like it may be one of the O-rings (#5 and #6 in your diagram) that sit under the clutch case.
Have you had that off? Was the O-ring just found loose in the sump?
If it is missing from the clutch cover, and wasn't replaced, you would expect a leak from the bottom of the cover.
#5 and #6 are the I/O for the oil pump. You would have had to have the pump off to have them fall out. Also, without them, the pump will leak and you wouldn't have oil pressure. They're also thick o rings, the one pictured looks very thin.
I believe its #12 on this picture. That's the o ring which will fall out when you remove the right side case cover.
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