The Sestertius denomination
The Sestertius was an ancient Roman coin. During the Roman Republic it was a small, silver coin issued only on rare occasions. During the Roman Empire it was a large brass coin.

The name sestertius means "two and one half", referring to its nominal value of two and a half asses, a value that was useful for commerce because it was one quarter of a denarius, a coin worth ten asses. The name is derived from semis, "half" and "tertius", "third", in which "third" refers to the third as: the sestertius was worth two full asses and half of a third.

The brass sestertius typically weighs in the region of 25 to 28 grammes, is around 32–34 mm in diameter and about 4 mm thick.

Sestertii continued to be struck until the late 3rd century, although there was a marked deterioration in the quality of the metal used and the striking even though portraiture remained strong.
Sestertius

(1) Rome | Anonymous 211-210 BC
Obverse: head of Roma right wearing winged helmet; IIS
Reverse: Dioscuri on horses riding right holding spears and reins, stars over pilei; ROMA
Ref: Crawford 44/7