Antique English Bloodstone/Carnelian Swivel Fob Necklace

This antique English sterling silver and stone circular swivel fob makes an exquisite necklace. The green side is probably bloodstone and the deep red stone is probably carnelian. The stones are set in silver and attached on the sides to a filagreed top. The stones spin smoothly. I’ve tried to capture the two hallmarks on the outside of the circle, but they are difficult to photograph. I suspect they might reveal the place and date it was made - probably 19th century Birmingham. I added an 18 inch sterling chain through it’s suspension loop so it could be worn as a necklace. A prize little piece of history.