The 1856 Sydney half sovereign with the Type 2 reverse (HST256R2) was for several years unique within the Australian gold coin series - no other circulating Australian half sovereign was known with this exact reverse design.
That all changed in April 2012 however, when an eagle-eyed numismatist spotted an 1861 Sydney half sovereign that had the same reverse design.
The 1856 Sydney half sovereign with the Type 2 reverse is extremely rare in any condition - research as at July 2016 indicates that possibly around 12 unique examples are known to be available to collectors.
The 1861 Sydney Half Sovereign with the Type 2 reverse is in fact quite a bit rarer - just 4 unique examples have been sighted on the open market to date.
The current body of knowledge on the background of the 1856 Sydney half sovereign with the Type 2 reverse has evolved over time:
November 1981 - The coin was first published in the Spink Noble Sale 39 auction catalogue in November 1981. Thought to have been produced by the same matrix used to strike the 1853 Sydney Mint pattern half sovereign - one of the most important coins in Australia's numismatic history.
2005 - Research conducted in preparation for the auction of gold coins taken from the Reserve Bank's archives showed that the reverse die used to strike the 1856 Sydney half sovereign with the Type 2 reverse was in fact quite different to both the 1853 pattern half sovereign, as well as all other Sydney Mint half sovereigns.
2012 - the first 1861 Sydney Half Sovereign with the Type 2 reverse was identified on eBay by a WA numismatist
That the 1856 half sovereign with the Type 2 reverse was struck with a pattern reverse die is beyond dispute, whether you believe this came about by accident or design depends on your view of the organizational culture and working conditions at the Sydney Mint during these formative years.
One logical explanation put forward regarding the existence of 1856 half sovereigns with the Type 2 reverse is that a further "pattern" half sovereign reverse die (i.e. one different to the die used to strike the 1853 patterns, and also different to those used to strike coins for circulation) was somehow unintentionally mixed with a batch of "working" half sovereign dies sent from London, and that therefore it was used quite by accident.
A more pragmatic thought is that the Sydney Mint may have been short of half sovereign reverse dies at some stage during 1856, and that the pattern die was deployed into the production process out of necessity.
It is likely that Sydney Mint staff found themselves in the same situation in 1861, which led to the existence of this particular example.
These circumstances mean the 1861 Sydney Half Sovereign with the Type 2 reverse is another rare and important reminder of the lengths that Sydney Mint staff were forced to go to ensure that production of Australia's earliest half sovereigns could continue uninterrupted.
In that sense, it ranks alongside the 1856 Sydney half sovereign with the Type 2 reverse, the 1858 Type II Sydney Mint half sovereign with the RR reverse legend "error", as well as the overdates known in the Type 2 Half Sovereign series.
As time passes, I expect further research will be done into the relationship between the die states of the different Type 2 reverse Sydney Mint half sovereign, which will increase clarity regarding the likely reasons behind their existence.