Morgan Dollar was USA first standard silver coin. It was minted from 1878 to 1904 and then again in 1921. The coin was named after its designer George T Morgan. The observe depicts Liberty portrait, while reverse depicted eagle with open wings. Due to shortage of silver, the mint ceased minting coin in 1904.
Then again in 1918, Pittman Act authorized melting and recoining of millions of silver dollars. Pursuant to the act, Morgan Dollar minting started again in 1921. From 1878 to 1904, the Dollar was stuck at Philadelphia, Carson City, San Francisco and New Orleans.
Denver Mint, which opens its gate in 1906, never stuck a Morgan Dollar since it was stopped in 1904. So when Morgan’s mintage started again. It was stuck at all the old mint plus Denver Mint, but since it was replaced by Peace Dollar replaced Morgan later that year, limited number of coins were minted at Denver Mint and they can be easy recognisable as “D” mint mark above Dollar in Reverse.