These coins were produced when the Royal Mint accidentally used a wrong obverse die when minting the 1960 proof sets.  At first glance, the incorrect die appears to be the one used on the 1945 Liberation issue.  However, the Royal Mint destroyed these dies when the new design was introduced in 1957.  The incorrect die is a British Caribbean Territories two cents die. The British Caribbean Territories two cents coin was the only coin of the same size, weight and alloy of the Jersey penny and incidentally had the inscription below the effigy.  Although proof specimens of the British Caribbean Territories two cents coin and the Jersey penny were struck at the Royal Mint in each of the years 1961, 1962, and 1963, there was only one time, viz. from August 8th through 22nd, 1961 when obverse dies of both coins were held in the coin processing room die store.  Evidently it was during this time period when the Royal Mint manufactured eighteen two cents proof coins for the Mint Museum, that this die was inadvertently used for the Jersey coins.  As for the number of coins produce, an exact number is impossible to determine.  However, it would be reasonable to assume that some coins were produced after August 22 and until late September when the order was completed.  Also, we can assume that some coins were produced before August 8 and that striking continued concurrently with the striking of the Caribbean two cents coins.  These mule coins are a tiny sub-set of the total mintage and thus, this coin is a modern rarity.