taken form the Royal mint museum facebook page, and what a beauty this coin is, almost certainly unique
The coin illustrated here is a piedfort sixpence of 1963. It is the same diameter and carries the same designs as a standard sixpence but at 6.460g it is more than double the normal weight and it is made in sterling silver. For many years it has been in the Mint collection but so far it has not been possible to determine precisely why it was struck. There is no evidence of relevant experiments taking place at that time and in terms of size it cannot be related to any other coin the Mint was making in 1963. For the time being, therefore, the coin remains a mystery but the search will continue for an answer.