Makers Mark db or Frontwards d backwards d encircled

by Diane F
(Homosassa, FL USA)

The piece is a money clip, marked STERLING & 14K above the Maker's Mark as I mentioned in the title of my post. Either db or frontward and backward d -- I'm not sure. The circle around it starts and ends with the top of each letter and does NOT join in the middle. I'm attaching a photo for easier ID. The gold is wrapped around the sterling silver like a rope. Very nicely done.

I've found all the marks I've had in question over the past 5 years on the internet, but THIS ONE has me stumped!!! Any suggestions would be GREATLY appreciated! Also, what is everyone's favorite book for marks on Fine Jewelry. I have lots for costume, but always use the internet for sterling and gold. Thanks! Diane