Bowers and Merena Galleries/AP Photo of a rare 1913 Liberty Head nickel, one of five in the world. CONCORD, N.H. -- A nationwide bounty hunt is under way -- with a $1 million reward. The target: a 90-year-old nickel. After being born of questionable, some say clandestine, circumstances, five 1913 Liberty Head nickels surfaced in the 1920s. Two are in private collections, two are in museums, but the whereabouts of the fifth has confounded collectors for at least 40 years."There's a little bit of gimmick to it," concedes Paul Montgomery, president of Bowers and Merena Galleries of Wolfeboro, N.H.,...