MLA

Wharton, Edith, 1862-1937. Bromfield, Louis. Box 23, folder 726. n.d.. https://collections.library.yale.edu/catalog/16782177.

APA, 6th edition

Wharton, Edith, 1862-1937. (["Box 23, folder 726"])(n.d.)[Bromfield, Louis]. https://collections.library.yale.edu/catalog/16782177.