The reference is to the number in the Breen Encyclopedia. In 1858 the half dollars from Philadelphia come with either the type 1 or type 2 rev hubs the type 1 are 4880 and the type 2 are 4881. The 4881 is the rarer of the two and he only lists it as coming from one die pair. The other die pairs are all type 1. And I agree yours is a type 2 rev. Breen calls the 4881 very rare, but take that with a grain of salt, many if not most of his rarity estimates were made in the 1950's when very little variety collecting was going on and many of his high rarity estimates have come way down.