Jerry, There are some vids here and on youtube showing Johnny playing and from what I see, he's using a bone or some other stick like surffrogg says (movable nut) that he moves up and down as he plays. sorta like barreing the strings in a way. I would say a stick-like instrument that is triangular in shape. Best guess.
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Hmmm, that's not a bad idea.
-WY