Struck as part of the celebration of the 100th anniversary honoring the transition from the Morgan to the Peace silver dollar. They were struck in .999 fine silver instead of .900 due to congress authorizing the change in silver content in US coins struck for collector and commemorative issues starting in 2019. Here is the relevant law.

Public Law No: 116-286 (01/05/2021)

1921 Silver Dollar Coin Anniversary Act

(Sec. 3) The Department of the Treasury shall mint and issue $1 coins in recognition of the 100th anniversary of completion of coinage of the Morgan dollar and the 100th anniversary of commencement of coinage of the Peace dollar.