From the record
“The Buffalo Gap Cheyenne River Bridge, in Custer County, South Dakota near Buffalo Gap, South Dakota, was built in 1932. It was listed on the National Register of Historic Places in 1988.”
Wikipedia article Buffalo Gap Cheyenne River Bridge, CC BY-SA 4.0.
One of 35 National Register listings published here for Custer County · listed in 1988 · category: Structure.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service