The extremely rare 1818 8 over 7 Half Sovereign is sometimes mis-identified due to existence of a variety that appears as if it could be due to a defect on the right side of the second 8, making that side of the coin appear fatter. There is however no trace of the upper side of the 7 which would be clear if this was indeed the rarity. This article illustrates the two types.
The George III 1818 8 over 7 Half Sovereign – An often mis-identified rarity. Graham Warboys.
Really useful article, thank you.