I found a copper Roman sestertius, but it has no markings:
weight = 12.38g, diameter = 27.3 to 27.8mm, thickness = 3.33mm
Sestertius (semis-tertius) means 2½, the coin's original value in asses, and is a combination of semis "half" and tertius "third", that is, "the third half". The first half was 1½, the second was 2½.
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