I have a bracelet that was my grandmothers. It is probably about 90 years old, a gold hoop, with delicate filigree, that has the sliding bar that goes inside the bracelet to latch it. On the inside of the bracelet is a marking that is hard to read. I think it says 1/20 FMC? or it could be 1/20FMO? It is in the original box from a jewelry place in Cleveland, that I guess is no longer around. What do you think this marking means?

Thanks