Found at Varia, via A Journey Round My Skull's tumblr.
Addendum Dec 4: The answer from Fester was so thorough that it seems appropriate to add it above the fold:
That is Sam Clemens back when he was a printer's devil.
That is not a buckle with his name on it. That is a compositor's block with three pieces of type on it. Looks to be about 60 to 72 point type.
Looking at loose type face on, the letters are backwards. However, this photo is a tintype which is a mirror image of the subject. Sam set the letters in order so they appeared right way around on the photo. Clever lad.