Should you ever come into possession of one of these with a 1913 mint date on it, keep it. There are only 5 known to exist and the last one sold for $5 million. One coin site notes the following:
Not much is known about the actual minting of the 1913 Liberty Head Nickels. It is believed that five specimens were struck at the U.S. Mint in Philadelphia sometime between the Summer of 1912 and early February of 1913. One theory says that the coins were struck as advance test pieces around July of 1912, with the expectation that the series would continue the following year anyway. Another theory proposes that someone was burning the midnight oil at the Mint, and struck the five specimens before the dies were destroyed in preparation for the change to the Buffalo Nickel, which began production in late February of 1913.