"The Gold Half Sovereign" by Marsh says:
"Though William IV succeeded his brother on the throne on 26th June 1830 no currency gold coins were issued for that year. An Order in Council, dated 22nd November 1830, did however specify that a double sovereign, sovereign and half sovereign be struck in gold. These became current by proclamation on 13th April 1831, although the double sovereign was not issued.
The obverse was engraved by William Wyon from a superb bust of King William IV by Sir Francis L. Chantrey. The reverse features a magnificently garnished shield modelled and engraved by Jean Baptiste Merlen.
The overall design differs slightly from that of the sovereign, the main difference being seen in the diminishing panels that appear in the lower garnishment on either side of the shield of both coins. On the half sovereign these panels are filled with diagonal lines giving a shaded appearance, whereas on the sovereign small round pellets are shown in this area."