MLA

Luhan, Mabel Dodge, 1879-1962. Brett, Dorothy. Box 71, folder 1922-1923. n.d.. https://collections.library.yale.edu/catalog/16683603.

APA, 6th edition

Luhan, Mabel Dodge, 1879-1962. (["Box 71, folder 1922-1923"])(n.d.)[Brett, Dorothy]. https://collections.library.yale.edu/catalog/16683603.