Hi Lucy, the ring sounds gorgeous. We have a Tiffany & Co ring featured on Jewel of the Month for July. It's a few years younger than your antique gold ring.
You mentioned a gap in the back? If the ring has been re-sized it can sometimes mess-up the marks.
It's a difficult one. I found two possibilities:
Hallmark Watch Corp
Hallmark Silversmiths Inc. (New York, NY (Active since the 1910s. Merged in 1954 with Hunt Silver Co Inc to form Hunt-Hallmark Co).
