I have two silver colored pennies. A 1973 D and a 1974 (no mint mark). These look and feel like regular pennies. Only difference is the sound they make when clinking together, almost a silver sound. Just wondering if anyone knew what these may be. I have heard of the 74 being struck in aluminum, but not the 73. Very interested in finding out what these are considering I have no idea why someone would copy a penny.