2008 $1 Silver Proof Subscription Coin-1921 Kookaburra Penny. This 2008 Subscription Coin from the Royal Australian Mint is based on a coin that was never in general circulation, and therefore is highly valued by collectors. The Kookaburra Pennies were the result of a 1919 Government decision to trial the introduction of square 18mm, cupro nicked coins instead of the heavier bronze pennies. This proof quality coin incorporates the original kookaburra design struck in pure silver. Certificate of authenticity included. This coin number 2939 and 2940 Mintage 6592.