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This is an ancient Roman gold solidus of emperor Valentinian III dating to circa 440 - 450 AD
Obverse: DN PLA VALENTINI-ANVS P F AVG, "Our Lord Placidius Valentinianus, Pious and Blessed August" rosette-diademed, draped, and cuirassed bust right
Reverse: VICTORI-A AVGGG, "Victory of the Augusts" Valentinian standing facing, holding long cross and Victory on globe, foot placed on human-headed coiled serpent.
R-M either side for Rome and COMOB in ex. *
See also Valentinian III (RIC X 2001-2187) also the last photo of a similar coin from the same die sold by CNG with this very distinct portrait.
This is a superb example in about EF condition.
*The moneta comitatensis was a travelling mint attached to the imperial train wherever the Emperor was situated. 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 Rome.
DATE: 425 - 455 AD (this coin minted in Rome between 440 - 450 AD)
MINTMARK: RM (Rome, Italy)
SIZE: 22mm diameter
WEIGHT: 4.47 grams
ATTRIBUTION: RIC X 2014; Depeyrot 46/1; DOCLR 850.
PROVENANCE: Formerly in a private collection Buckinghamshire, United Kingdom