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This is an ancient Roman gold Solidus of emperor Theodosius II, who reigned from 408 - 450 A.D.
Obverse: D N THEODOSIVS P F AVG, "Our Lord Theodosius, Dutiful and Wise Augustus" helmeted, cuirassed three-quarters-facing bust, spear over shoulder and shield decorated with horseman spearing enemy.
Reverse: GLOR ORVIS TERRAR, "Glory (of Christianity) over the Whole World" Theodosius standing. facing holding labarum and globus cruciger, star in the field.
This coin was struck at the Constantinople mint and is datable to the years 424 - 425 AD. It bears the Officina letter S (for SECVNDA) this indicates the coin to have been minted by the 2nd Officina at the Constantinople mint. Mintmark CONOB. This very short-lived issue of solidi was likely connected with providing the funds for a campaign in Italy aimed at installing Valentinian III on the throne of the Western Roman Empire.
DATE: 408 - 450 AD. (this coin minted between 424 - 425 AD)
MINTMARK: COMOB (Constantinople, Turkey)
SIZE: 21mm diameter
WEIGHT: 4.38 grams
ATTRIBUTION: (RIC 257; Hahn 25b).
PROVENANCE: Formerly in a private collection. Frankfurt am Main, Germany.