MLA

Dellenbaugh, Frederick Samuel, 1853-1935. Montana. Box 1, folder 20. n.d.. https://collections.library.yale.edu/catalog/2017625.

APA, 6th edition

Dellenbaugh, Frederick Samuel, 1853-1935. (["Box 1, folder 20"])(n.d.)[Montana]. https://collections.library.yale.edu/catalog/2017625.