Could be a British registration mark. From 1842 through to 1867 the mark has the shape of a diamond with a circle at the top. The circle should have the Roman numeral (capital i) designating that the piece is made of metal. After 1867 the circle was omitted...
Richard, the date is probably 1883 (?i? in circle) however it depends on the shape of the ?i? . If you have a photo, it would be easier to determine the exact date.
