The 1856 Alternate Reverse Half Sovereign, first identified in a Spink Auction in 1981 (Crellin, A, 2015) but incorrectly attributed as having the 1853 pattern reverse is one of the more interesting varieties to come out of the early days of the Sydney mint. Its origin and purpose is a mystery but given the Sydney mint's constant shortage of dies as evidenced by the amount of tooling done to extend their working life in many of the early years, it is likely that this die was some sort of pattern or discarded die that was put into use to satisfy the colony's demand for gold coin.
The best defining feature is the lack of a berry in the wreath at 7 o'clock, there are other subtle differences in the wreath and bow as well as seen below:
One thing is for sure though, this variety is rare with just one confirmed example prior to the release of the Reserve Bank gold holdings with offered just three more to the marketplace (Downies, 2005) . Since then an additional eight have been identified bringing the total examples known up to 12 (Crellin, A, 2015) . Almost all of these pieces are heavily circulated though one XF-AU level piece is known.
Considering the survival rate of the era of about 1 in 400 coins, this piece probably had a mintage of about 5000 pieces ruling out it being a pattern striking as was previously thought though it is still no doubt extremely rare.