20 Cents - Elizabeth II 4th Portrait - Platypus - Fine Silver Proof

KM#403a tracks a long run of annual proof issues, but the series has a specific origin worth noting: the Royal Australian Mint introduced fine silver proof versions of the circulating denominations in the 1990s partly in response to collector demand following the transition to the aluminium bronze dollar and two-dollar coins a decade earlier, which had effectively ended silver's presence in everyday Australian commerce. The 20-cent denomination had carried silver composition through 1966–1974 before the shift to cupro-nickel, making the proof reissue in .999 a deliberate nod to that earlier standard.

Ian Rank-Broadley's fourth portrait of Elizabeth II, adopted by the RAM in 1999, means pieces dated 1998 carry Stuart Devlin's third portrait — a one-year distinction within this KM number.