It is a watch fob seal. They were attached on the chain of a watch fob and were supposed to be used to make an impression on wax seals that were used on documents and envelopes. They were popular in the late 1800's and fell out of use about 1920. I think they were more for show than actual use. I found an example somewhat similar to yours.(without the chain) The photos are not meant to hi-jack your post, only to show you another dug example. Yours has a nice engraved letter on the bottom. Nice find !