The Rarest Error Gold Sovereign
The 1880 Sydney Shield reverse sovereign with the inverted "A" error in the obverse legend is a true rarity within the Australian sovereign series - it is one of just six errors known on Australian sovereigns, and is one of just two Australian sovereigns that have an error in the legend.
The tiny population of this important sovereign is in fact equal to a number of the rarest Australian sovereigns - just 15 have been sighted at auction since the first example was discovered in February 1998. Interestingly, it was not listed in Greg McDonald's Guide to Australian Coins and Banknotes until the 8th edition, issued late in 2000.
It is worth noting that none were offered via auction until July 2004 - some six years after the first example was discovered, and that there is undoubtedly some duplication within those 15 auction appearances.
The 1880 Sydney Shield reverse sovereign with the inverted "A" error is a highly prized rarity in any condition, and reminds us of the regular human input that was fundamental to the unique process by which coins were manufactured at the Royal Mint in the 19th century.
The famed craftsman Leonard Charles Wyon was the Chief Engraver at the Royal Mint for the latter part of the 19th century. As such, many of the dies used for striking the gold, silver, copper, bronze and nickel coins of different denominations which were issued for currency in various parts of the British Empire after the death of William Wyon [in 1851], were prepared by Leonard Charles Wyon, either from his own designs, other artists' models, or drawings supplied to him by the British and some of the British Overseas Governments. 
By extension therefore, the 1880 Sydney Shield reverse sovereign with the inverted "A" error clearly remains a direct link to one of the world's finest die engravers of the 19th century.
An auction conducted in London in September 2007 by Spink and Son, of wax models, patterns and other a number of other important items relating to the Wyon family, drew bidding and worldwide attention, several of those lots fetched many thousands of dollars.
Footnote,  Leonard Forrer. Biographical dictionary of medallists: coin, gem, and seal engravers, mint-masters, etc. volume 6] (London: Spink and Son, 1904)