When him and the guys he was on tour with where in Europe last year they played tattoo roulette. They took the 6 murder weapons from clue and gave each  a number and then put the numbers in the hat. As they blindfolded themselves they picked a number that was the tattoo they got. Frank got a pistol it's by frankensteins leg.

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Thanks to HeatherWescott for submitting in this fact about Frank.