The U.S. didn't mint any silver dollars in 1920.
The 1920 Half Dollar - aka - Liberty Walking - mint mark of "D" or "S" would be located on the reverse side, just to the left and above the "H" in half dollar.
Mintmarks are on the reverse above the letters DO in dollar
San Francisco
The mint mark on all Morgan silver dollars is located on the reverse side of the coin just above the letters "DO" in the word "DOLLAR".
The mint mark on all Morgan silver dollars are located on the reverse of the coin near the bottom. Look above the "D" and "O" in the word DOLLAR
The mint mark on any Morgan dollar is located on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
1899 would make it a Morgan dollar. The mint mark is located on the reverse (tails) side, near the bottom, just above the letters "do" in the word "dollar."
By the mint mark on the reverse of the coin, but silver dollar coins with no mint mark are made in Philadelphia
The mint mark on the 1974 Eisenhower silver dollar is located on the reverse side of the coin, just to the right of the eagle's tail. If the coin was minted in Philadelphia, it will have no mint mark. If it was minted in Denver, it will have a "D," and if it was minted in San Francisco, it will have an "S."
1928 makes it a Peace dollar. The mint mark is located on the reverse (tails) side, below the word "one." D for Denver, S is San Francisco, and no mark is Philadelphia.
above the DO in dollar on the back :]
The mint mark on any Morgan dollar is on the reverse above the letters DO in dollar.