The following is excerpted from Clifford Mishler’s Coins: Questions & Answers
Q: Where is the mintmark located on the 1916 Liberty Walking half-dollar?
A: The Denver and San Francisco mintmarks on 1916-dated halves, and the early portion of their 1917 productions, are located on the obverse immediately below the “IN GOD WE TRUST” motto. This marked the first appearance of mintmarks on the obverse of the half-dollar since the 1838 and 1839 Liberty Cap issues struck at the New Orleans Mint carried its identifying “O” immediately above the date. In 1968 the Kennedy half-dollar became the third coin of that denomination to bear a mintmark on its obverse, at which time mintmarks were also moved to the obverse of nickels, dimes, and quarters as well.