I note that the date is 1981. I am not a specialist on Australian decimals, therefore I have a question:
Did the R.A.M. contract out any of the 1981 5 cent production to a non Auasralian mint, e.g. the Royal Mint
, or the Royal Canadian Mint?
If the answer is 'yes', especially if it is the Royal Mint
, it is highly probable that a blank from another country could have got mixed up in the order for the Australian coins.