I believe that it's the Marine insignia, like this. Hold on while I check.... No, I think I was wrong. I can't make it out all the way, but if I had to guess, I'd say it was a Grim Reaper. Or a gun, it's hard to see clearly, and I don't think it shows up again. *squints* I can't tell.

You could probably get away with whatever you wanted to put there.