Where Is a Mint Mark on a Dime?

By GovMint :

The widely collected Roosevelt dime has been in production, with the design unchanged since 1946. Well, it is intact, except for the placement of the mint mark. The original design saw the mint mark placed on the reverse at the lower left of the torch. The identifying mark remained there until 1964. Then, in 1968 the mint mark was moved to the obverse of the coin, placed just over the date. 

 In 1980, the P mint mark appeared on the dime, as it did on everything larger than the nickel. The intention was to have the P mint mark appear on all future Philadelphia struck coins.

Sources Used:

https://www.interestingfacts.com/fact/6386d2b1bd1439000808d979

Concept of the Month: Fun Facts About Coins - Young Americans Center (yacenter.org)