MLA

Luhan, Mabel Dodge, 1879-1962. James, Rebecca Salsbury. Box 41, folder 1220. 1929 Sep 19. https://collections.library.yale.edu/catalog/17085372.

APA, 6th edition

Luhan, Mabel Dodge, 1879-1962. (["Box 41, folder 1220"])(1929 Sep 19)[James, Rebecca Salsbury]. https://collections.library.yale.edu/catalog/17085372.