From the record
“Duncan Church, also known as Saint Placidus Catholic Church, is a historic church near Gann Valley, South Dakota. It is located southwest of Crow Creek, about 2 miles (3.2 km) south of the county line.”
Wikipedia article Duncan Church, CC BY-SA 4.0.
One of 3 National Register listings published here for Buffalo County · listed in 1999 · category: Building · built in 1927.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service