User blog comment:Phoenixs23/Natsu's Arm (175)/@comment-94.201.237.192-20130403110210/@comment-7418318-20130403131406

That's no insignia, those are the shadows that his fingers cast onto his palm. Natsu's guild stamp is on his shoulder.

As far as I remember only Lucy and Freed have their stamps on the back of their hand and Lucies stamp is on her right hand, not her left.