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This is an ancient Roman gold solidus of emperor Arcadius dating to 394 - 408 A.D.
Obverse: DN ARCADI-VS PF AVG "Our Lord Arcadius Pious Fortunate August" pearl-diademed draped and cuirassed bust right.
Reverse: VICTORI AAVGGG "To the Victory of the Emperors" Arcadius standing right, foot on captive, holding standard and Victory who crowns him. M.D. either side for Milan and COM OB in ex. *
*The moneta comitatensis was the imperial court mint that followed the emperor. A special mintmark COM for comitatus (or COMT.M) Most coins signed by the comitatensian mint also bear a supplementary abbreviation referring to the name of the town where the coin was actually struck in this case in Milan. The OB refers to pure gold - so it would read as "pure gold of the Imperial Court" (COMOB)
DATE: 394 - 408 A.D.
MINTMARK: MD (Milan, Italy)
SIZE: 20mm diameter
WEIGHT: 4.34 grams
ATTRIBUTION: RIC IX Milan 35b; Sear 20724.