MLA

Sergeant, Elizabeth Shepley, 1881-1965. Benét, William Rose. Box 1, folder 10. [1921]. https://collections.library.yale.edu/catalog/10520327.

APA, 6th edition

Sergeant, Elizabeth Shepley, 1881-1965. (["Box 1, folder 10"])([1921])[Benét, William Rose]. https://collections.library.yale.edu/catalog/10520327.