This is best identified by noticing the 1942 date punched over a 1941 date. This error from the Denver Mint is also very noticeable but not as much as the same error that came from the Philadelphia Mint. The 1942 42 over 41 Overdate Error (1942/1-D) was one of the major errors in the silver Mercury Dime series. What This Coin Looks Like (Obverse, Reverse, Mint Mark Location, Special Features, etc.):
