What this coin looks like (obverse, reverse, mint mark location, special features, etc.): The 1942 42 over 41 Overdate Error (1942/1-P) was one of the major errors in the silver Mercury Dime series. It is also one of the most identifiable and prominent errors in all US coins. The error from the Philadelphia Mint is far more noticeable than the same one at the Denver Mint. This is best identified by noticing the 1942 date punched over a 1941 date. The underlying "1" is very noticeable on the left-hand side of the "2" and part of the doubled "4" is also visible towards the bottom of the "4" as depicted by the arrows in the example image above.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition