The Type I twenty cent was issued from 1966 to 1984 with an additional
striking in silver in 2006 as part of the 40th anniversary of
coinage set. The reverse features Stuart Devlin's platypus design and the obverse features Ian Rank-Broadly's portrait of HM Queen Elizabeth II.
The issue was struck at the Royal Australian Mint in all years with additional assistance provided by the Royal Mint London in 1966 to address the huge demand for new currency as a result of the vast amount of
currency withdrawn. To distinguish between the 1966 London and Canberra coins one must look closely at the point of the wave near the platypus' head. There is no gap (or a very small gap) between the swirl and the head it is the London coin and a distinct gap on the Canberra one. Unfortunately a slightly weaker strike on one of the London minted coins can lead to a small gap causing confusion.
In 1981, the Wales mint assisted with striking coins to meet the additional demand for circulation currency brought about by the economic boom of the 1980s.