1904 Perth Half Sovereign
At the start of the 20th century in Australia, half sovereign mintages were not particularly high, and the 1904 Perth half sovereign was no exception, with just 60,030 coins recorded as being struck.
Marsh records two reverses as having been used to strike the 1904 Perth half sovereigns (p64, Marsh, The Gold Half-Sovereign, 2004) - reverse A and reverse B - with the easiest difference to spot being the absence of the designer's initials B.P. on reverse A coins and the presence of the designer's initials B.P. on the reverse B coins.
Although both reverses are reported to have been used on 1904 Perth half sovereigns, it appears that only reverse A was used in reality. The author has not seen any examples of a reverse B coin and McDonald lists the "BP" coins as "Not verified" but reported (p54, McDonald, Pocket Guide to Australian Coins and Banknotes, 2014). It is unclear if Marsh ever sighted any examples either - in his book the reverse A coin is numbered as Marsh 518 and the reverse B coin is numbered as Marsh 518A (p64, Marsh, The Gold Half-Sovereign, 2004), which, while not conclusive, suggests it was newly discovered and at least reported to him. In any case, it seems unlikely that reverse B was used on 1904 Perth half sovereigns.