1920 Sydney Sovereign
The 1920 Sydney sovereign is one of the rarest circulating sovereigns in the sovereign series. There has been much speculation about its rarity. Although mint records showed 360,000 coins as being struck - not many, but not an overly low amount either - there are only a handful of examples known. Conventional wisdom was that almost all of the mintage was melted down at some point in the 1920s or 1930s, along with many other sovereigns after World War I.
The sale of The George Collection of sovereigns in 2014 attempted to shed new light on the matter, with research finding all sovereigns struck at the Sydney Mint in 1920 were most likely dated 1919, and that prominent New South Welshman Jacob Garrard had specially ordered a small number of 1920S sovereigns to celebrate his golden wedding anniversary in 1920. The specially ordered sovereigns were to be given to his children, and possibly himself and his wife (https://www.sterlingcurrency.com.au/blog/news-research/australian-gold-coinage/the-1920-sydney-sovereign-the-greatest-gold-rarity/).
More recent research by Howard Hodgson suggests that the previously believed story of mass-melting of sovereigns after World War I is indeed correct (p28, Hodgson, The 1920 Sydney Sovereign A Centenary Review in Australasian Coin and Banknote Magazine, March 2021). He concluded that Jacob Garrard did likely obtain a small supply of 1920S sovereigns, and that this supply likely accounts for most or all known surviving specimens. Also of note is that Hodgson discovered that descendants of Jacob Garrard have in their possession two previously unaccounted for 1920S sovereigns (p29, Hodgson, The 1920 Sydney Sovereign A Centenary Review in Australasian Coin and Banknote Magazine, March 2021). With five previously known examples, this brings the total known number of 1920S sovereigns to seven, with possibly more unaccounted for examples in the possession of other Garrard descendants. In spite of this, the 1920S sovereign still remains the rarest sovereign in the series.