From the years 1938 to 1964 the mint mark was on the reverse on the far right next to Monticello. This is the rule with an exception made for wartime nickels minted from 1942-1945. Wartime Nickels mint mark is on the reverse top center, only wartime nickels have an actual P mark. After 1964 the mint mark was moved to the obverse (front) of the Nickel. Source: https://coinbin.com/info/jefferson-nickel/
