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One side was inscribed with the arms of George I above the cipher GR, dating it to c.1720. However, the decoration to the other side hints at a more specific commercial function. It shows a post boy on horseback blowing his horn while above flies Mercury holding his caduceus in one hand and a letter in the other.

The background street scene features a distinctive tiered steeple which could be that of St Brides in the City of London.

This led Sotheby's to surmise that the plaque is related to the London Penny Post established in 1680 by William Dockwra, taken over by the Government two years later and absorbed into the Post Office.

Dockwra used a system of sorting stations and receiving offices manned by innkeepers-cum-postmasters to take in letters and provide fresh horses for the post boys who were able to make several deliveries a day in the city at the cost of a penny and up to ten miles beyond for an extra penny.

Mr Horne admitted that he was not sure of the plaque's specific function. Its very rarity means it has no precedent to give any clue.

"We are looking at a minute fragment of what has been made," he explained.

The two-sided decoration means it probably wasn't set in a wall. The suspension holes, he speculated, suggest it could have served as a sign or perhaps have been mounted on a pole for some ceremonial or official function. The plaque has now gone into a private collection so perhaps future research will solve the mystery.

By Anne Crane