Silver Proof 10p

To commemorate the introduction into circulation of a new, smaller ten pence coin in 1992, the Royal Mint issued a limited number of silver Proof and silver Proof Piedfort ten pence coins.
 
Piedfort coins are struck on thicker than normal blanks.  As their name suggests they are closely associated with France, where from the twelfth century they were apparently used by the King as presentation pieces.