From the record
“The Bank of Buffalo County, on Main St. in Gann Valley, South Dakota, is an Early Commercial-style building built in 1921. It was listed on the National Register of Historic Places in 1999.”
Wikipedia article Bank of Buffalo County, CC BY-SA 4.0.
One of 3 National Register listings published here for Buffalo County · listed in 1999 · category: Building · built in 1921.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service