MLA

Luhan, Mabel Dodge, 1879-1962. Brett, Dorothy. Box 65, folder 1749. n.d.. https://collections.library.yale.edu/catalog/2002406.

APA, 6th edition

Luhan, Mabel Dodge, 1879-1962. (["Box 65, folder 1749"])(n.d.)[Brett, Dorothy]. https://collections.library.yale.edu/catalog/2002406.