Guineas with the first bust were minted in 1695-7, however a new die was introduced in 1697 which featured prominent lettering and a more striking profile. This new obverse die was used with a new reverse in which the harp was given a human head. This combination of new dies was used for most guines dated between 1697 and 1701. A third reverse, with scrolled harp, was used in 1701, and a third bust with much finer detail was introduced in late 1701.
Half-guineas: Only one major reverse type was used, but there are two types of reverse, differing in the harp style.
All of William's gold coins may be found with or without the Elephant & Castle mintmark.
Reference: James Mackay: A History of Modern English Coinage.