It is the makers mark
An S inside a gold ring is the maker's stamp. If the ring were not gold and had an S inside it would stand for sterling silver.
That is the company or maker. Your ring is gold with the 14k on it.
The Maker's Mark
I just bought a ring with that stamping. It shows who the maker is. My ring is a gold with opals. What I don't know is who that maker is, does anyone?
14k gold pf is the maker
what does 18K SAC mean in a ring
The markings inside a ring almost always comprise of the initials of the jewelry maker and the actual gold content of the ring. In the example nk14k, the nk stands for the maker or manufacturer of the ring. While the 14k stands for the purity level of the ring. 14k gold rings are 58.5% pure gold and the remaining 41.5% are other non-precious metal alloys.
Weight of the gold or the p stands for the maker
it means your ring is 14k gold and the maker of the ring is Bluebird which is an antique late 1800's possibly ( my best guess).
Mark used by a Canadian maker of rings
14K is the content of gold in the item, and SD is most likely the initials of the ring's maker.