MLA

Butts, Mary, 1890-1937. The Salterns. Box 20, folder 299. [ca. 1900]. https://collections.library.yale.edu/catalog/9998450.

APA, 6th edition

Butts, Mary, 1890-1937. (["Box 20, folder 299"])([ca. 1900])[The Salterns]. https://collections.library.yale.edu/catalog/9998450.