These extremely rare bands are called "skeletals" - the first example dates from 1659 and this one was made not long after in 1731. A full skeleton and other death-themed emblems are meant to drive home the brevity of life. An hourglass is symbolic of time passing, and a crown on the skeleton's head reminds us that death is the ruler of us all. A coffin and gravedigger's tools are other poignant memento mori symbols that we recognize from tombstones and paintings from the era. There's significant loss to the enamel, which we actually like, because it means that you can truly wear this ring. A perfect example with intact enamel would be unwearable without great care. A similar one (in pristine condition) has been deaccessioned by the Museum of London and is available from the venerable Symbolic & Chase; another example can be found in the British Museum.