George IV 1828 Sovereign, key date for reign and second rarest of London series

George IV (1820-30), gold Sovereign, 1828, bare head left, date below, rosette either side, GEORGIVS IV DEI GRATIA, rev. crowned quartered shield of arms, BRITANNIARUM REX FID: DEF:, weight 7.98g (Bentley 21; Marsh 13 R4; S.3801). Lightly toned though seemingly once lightly wiped on one side, surface marks and digs both sides, otherwise very fine to good very fine, has been slabbed by NGC as reverse cleaned, of the highest rarity being the rarest London Sovereign after the 1819, key date for George IV.