The 1916-D Mercury Dime is the key date of the Mercury series with a mintage of 264,000 coins. In 1916, the Philadelphia Mint made 22,180,080 coins and the San Francisco Mint, 10,450,000. Those two other mintages are important because scammers began added a mintmark to Philadelphia coins that year. Many also altered 1916-S dimes to make that mintmark look like a D.