It appears to be a modified lobster claw clasp. Sometimes hallmarks are difficult to find.
I have also had pieces that had no marks. I believe that some pieces make it through production and don't get marked. You can get a test kit or have it tested at a jeweler.
Is the item a standard length? I have had items sized and the jeweler accidentally removed the part that was marked. If you find that it is 14k etc. a jeweler will engrave a mark for you.
