Mint mark: If none can be found it is (Philadelphia Mint), but many have the mint marks D (Denver Mint - 1921 only), S (San fransisco Mint), O (New Orleans Mint).
Note: This George T. Morgan creation has become America's most collected, famous, and abundant silver dollar. Coupled with the coin's size, silver content, beautiful design and high mintages it's no wonder it's coin collectors most favorite coin.
It's rare to find 1921 Morgan Dollars in proof like or deep mirror proof like. A true PL or DMPL 1921 will have an obvious contrast between the cameo (frosty) devices and the mirror like fields, and there should be at least a clear 2-5 inch reflection in PL, and 5-8 inch reflection for a DMPL.