It is.
L. A. Huffman was a famous frontier photographer in Miles City, Montana. The photograph below is marked, "L. A. Huffman, Miles City, MT" which stands for Montana Territory. This means that this photograph was taken prior to Montana becoming a state in November of 1889.
To add a sense of mystery, the photograph was cropped for cover of The Shopkeeper. Add professional typesetting some color adjustment, and you have a distinct cover.
The intent was to identify the book as a Western, but signal that it was a fresh take on the genre. I think the designer did an excellent job.