Brian Page said he unearthed this hammered silver coin from a ploughed field in south Lincolnshire. He had never seen anything like it so asked if I could provide further details.
The coin is a Venetian soldino. Over the years lots of these have been found by detectorists. They are fairly common but having said that I have yet to locate a single example.
On one side, within the inner circle, is the lion of Venice with a Gospel. On the other side is the kneeling figure of a Doge (the head of the Venetian Republic) holding the flag of Venice, together with a legend reading +MICHAEL STEN DVX. Therefore, this soldino was struck for Michael Steno, who was Doge from 1400 to 1413.
Soldinos were imported into England in large numbers and were used as small change. This was because there was a shortage of low denomination coins. Ships that docked at Galley Wharf in London brought in bags full of these small coins, so they were sometimes called galley halfpence. In its present condition this Venetian soldino might be worth £15 – £18.