I checked, and there's definitely no period after the 'D'. I understand what Ipcress means, that it's a fairly simple trademark and has probably been used more than once. There is a back story to the ring though, and taken together it makes some sense that the ring is possibly American.
[My great aunt's mother disappeared in 1903, and I have found out through a lot of research that she travelled to New York. My great aunt was the eldest child and age 14 at the time - she married in the UK in 1920 and left this ring (supposedly her engagement ring) to her niece when she died. And now it has been given to me. I didn't expect the ring to have a connection to the US, but it intrigues me that my great aunt may have had some contact with her mother over the years...]
Thanks again everyone, you've given me plenty of food for thought!
