The hallmark/symbol on the back of the clasp in your photos is one known as a 'jungin' (Japanese silver-purity mark) for sterling silver items .
It (the jungin) appears to be like those from around 1914 through 1928 or so , to me .
The nomenclature of the clasp & necklace itself indicate (to me , anyways) that it could have been a clasp that has been added to the piece @ a later date , though .
