1904 London Half Sovereign
Marsh records two reverse dies as having been used to strike 1904 London half sovereigns (p64, Marsh, The Gold Half-Sovereign, 2004) - reverse A and reverse B. There are a number of differences between the two but the easiest difference to spot is 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.
As with many such changes, the reasons are unknown, but 1904 was a transitional year for the half sovereign. Although Marsh lists the reverse A coin as more common (p64, Marsh, The Gold Half-Sovereign, 2004), it would appear that the reverse B coin is much more common, though insufficient examples have been sighted to estimate a break-down. Only reverse A was used on the Perth-struck half sovereigns that year and neither the Melbourne Mint nor the Sydney Mint struck 1904-dated half sovereigns. It is unclear how the dies were prepared and when but it appears that the Perth Mint used reverse A exclusively and the Royal Mint used reverse B to strike most of its coins: either existing old dies were sent to Perth, or the Perth Mint ordered its half sovereign dies before the introduction of the new design.