This information from Don Heath.

There is always considerable confusion between British and American shotgun pellet sizes, particularly as Africa uses the British system of nomenclature and the majority of visiting sportsmen, American. Hunters are often enough at sixes & sevens over what size to use on a particular species without the added confusion of pellet size nomenclature. I have seen too many brochures recommending (1's) for geese, and know full well that the operator means British 1's, but most clients will bring American 1's instead of 3's. I have also seen far too many rough shot size comparisons that are totally misleading. Here, SSG may be a preferred load for leopard, but I have seen SSG equated to anything from 00 Buck to Number 4 Buck (its actually No2). As a service to shooters the following table is provided. The shot size is given in inches, being taken from the current Winchester and Federal catalogues, (which actually concur) who should know about American shot sizes, and the Ely and Somchem catalogues who should know what they produce with regard to British shot size.