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This is an ancient Roman silver Siliqua, of emperor Julian II, dating to 360 - 363 A.D.
Obverse: FL CL IVLIANVS P P AVG, "Flavius Claudius Julian, perpetual August (Emperor)", rosette-diademed, draped, and cuirassed bust right
Reverse: VOTIS V MVLTIS X "vows (prayers) on the fifth anniversary (of the emperor's rule), and for his (anticipated) tenth anniversary" in four lines within wreath.
This coin was minted at Lugdunum (Lyons in France) mint mark LVG. It bears the 'S' (for secunda) for the 2nd Officina at Lyon (SLVG).
DATE: 360 - 363 A.D.
MINTMARK: LVG (Lugdunum, Lyons, France)
SIZE: 17.5mm dia
WEIGHT: 1.94 grams
Attribution: RIC VIII 219; RSC 163
PROVENANCE: Formerly in a private collection, Hertfordshire.