Identifying Jewelry Trademarks: What Does 'D.O.F.' Mean?

Question
I have a Tanzanite and Diamond Necklace that has the trademark D.O.F. stamped on the piece.  Can you tell me who the manufacturer is?

Answer
Sorry for teh delay, iut's been crazy busy here in the lead up to Xmas..........
I have answered this sort of question numerous times.
Thee is no way to know as there is no register.
I assume it's the mark of the maker, or perhaps it means Department of Finance.

Happy Necklacing, P