Based on what I can find it probably has the hallmark of OB, short for Ostby Barton Company. It appears to be from the 1930's-1940's. Fun fact, Ostby died on the Titanic which resulted in a massive shift in how the company was run. His children took over and managed to turn the entire company around. One of my first gold ring's is an Ostby Barton and my girlfriend loves flashing that ring and telling that story.