From the record
“Lost Horse Fireman's Cabin, in the vicinity of Darby, Montana, was built in 1938. It was listed on the National Register of Historic Places in 1989.”
Wikipedia article Lost Horse Fireman's Cabin, CC BY-SA 4.0.
One of 92 National Register listings published here for Ravalli County · listed in 1989 · category: Building.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service · the Wikipedia article