The 1954 penny is one of the mysteries of British numismatics.
Because of the large number of pennies already in circulation, and the economic depression after the Second World War, there was no need to produce any more penny coins in Great Britain in the 1950-ies. That said, a new monarch in 1952 sparked off a huge demand for coronation coin sets and, in 1953 over 1.3 million Queen Elizabeth II pennies were minted to make up these sets, in addition to allowing some to circulate.
As usual, a new portrait causes problems and the Royal Mint had to experiment with the design. These design experiments continued into 1954 and, although it was not planned to issue any pennies for that year, at least one "complete" penny dated 1954 was produced - apparently for private internal purposes at the Royal Mint. The mint sites a trial run of hundreds of coins, all of which were supposed to be destroyed. How that one "escaped" and found its way into the collection of C. Wilson Peck is unknown; furthermore, in response to a freedom of information request in 2011 the mint stated it was "unable to say for certain that only one survived from the trial run of several hundreds".
The experimental output included some uniface (struck on one side only) variants; two of these are held by The British Museum, and the other four are part of the Royal Mint Collection. | 
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