In late 1973 the cost of copper rose rapidly make it a real possibility that the cost of manufacturing a penny would rise above one cent. In an effort to resolve this dilemma the mint in Philadelphia made over 1.5 million pennies out of aluminum, dated 1974. Once test samples reached the hands of vending machine makers, it was decided that the change over would be too costly. Since the vending machines thought they were fake and would require major upgrades. All coins were supposed to returned to the mint and after being individually counted, melted. Thirteen of them remained outstanding, the owners mainly congressmen claimed they had lost them. One was preserved for the Smithsonian. If one came one to the market today it would fetch at least 5 figures, however its ownership in the U.S. is probably illegal. (see coins you cannot legally own)