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This is an ancient Roman silver Siliqua dating to the late 4th - 5th century A.D. The coin has been heavily clipped, removing all inscription of the emperor. part of the mint name is visible, likely MDPS for Milan in Italy - if so the emperor could be identified as Arcadius or Honorius.
This genre of siliqua is known for the period after the Romans left Britain, see 'The coin clipping phenomenon in sub-Roman Britain'. This coin was excavated in Yorkshire and is a classic example of this post-Roman "Arthurian" period coinage.
CULTURE: SUB-ROMAN / Arthurian
SIZE: 11mm diameter
WEIGHT: 0.56 grams
PROVENANCE: Found in Yorkshire