It's actually AW, this is the monogram of the designer A.A. Weinman and all Mercury Head dimes have it. It's not a mintmark. A 1940 Mercury dime is very common, if it's circulated the value is for the silver, about $2.00.
The mint mark is located on the reverse [tails] side of the coin at the bottom below the wreath. If there is no mint mark then the coin was minted at the Philadelphia Mint.
The M is not a mint mark but on every coin and is on the right side. The mint mark is to the left od the date slightly above.
The mint mark is located on the obverse (heads) side, just above the date and to the left of Liberty's legs. It was unusual for the mint mark to be on the front, as it was on the reverse of all other U.S. coins (except for Lincoln cents) until 1968.
Roosevelt dimes dated 1968 and after have the mint mark on the obverse (heads) side, just above the date.
On the reverse side near the eagle's tail. Philadelphia issues do not have a mint mark, only Denver.
The mint mark is located on the reverse [tails] side of the coin at the bottom below the wreath. If there is no mint mark then the coin was minted at the Philadelphia Mint.
The mint mark is near the bottom center of the side with the eagle. There might not be a mark.
The mint mark position is on the front of the coin on the left side, above her shoulder.
The M is not a mint mark but on every coin and is on the right side. The mint mark is to the left od the date slightly above.
Of what coin.
The mint mark is located on the obverse (heads) side, just above the date and to the left of Liberty's legs. It was unusual for the mint mark to be on the front, as it was on the reverse of all other U.S. coins (except for Lincoln cents) until 1968.
Roosevelt dimes dated 1968 and after have the mint mark on the obverse (heads) side, just above the date.
On the reverse side near the eagle's tail. Philadelphia issues do not have a mint mark, only Denver.
To clear things up, what you have is called a Peace dollar. The "lady" is Miss Liberty herself, as in "Statue of ..." The mint mark position is near the word ONE on the back: No mint mark = Philadelphia D = Denver S = San Francisco Please see the Related Question for more information.
The mint mark of the Denver Mint (shown as a "D") and the San Francisco Mint (shown as "S) on the Eisenhower Dollar is located on the obverse (heads) side of the coin directly beneath the bust of Eisenhower. If there is no mint mark there, then the coin was struck at the Philadelphia Mint and in this mint did not place a mint mark on the coins struck there until 1979.
The mint mark on Franklin half dollars is on the reverse side, just above the Liberty Bell.
The mint mark is on the reverse (tails) side, just below the eagle and above the the words ONE DOL. If there is no mint mark, it means the coin was minted in Philadelphia.