Figured out who manufactured my ring! The hallmark is actually "BDA" and not "BOA". Don't ask me how I did it, but I found out the maker was Budlong, Docherty & Armstrong of Providence, RI. They were in business from 1850 to 1971. They patented the "BDA" hallmark in 1943, but I guess they could have stamped it "BDA" for many years before that date. Mystery solved!