Silver Proof 50p

Since the introduction of the 50p coin in 1969, it has become one of the most collected coins, with specially minted commemorative editions being produced in both Silver Proof and Silver Proof Piedfort version.
 
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.