MLA

French, Jared, 1905-1988. French, Margaret. Box 5, folder 99. undated. https://collections.library.yale.edu/catalog/16629587.

APA, 6th edition

French, Jared, 1905-1988. (["Box 5, folder 99"])(undated)[French, Margaret]. https://collections.library.yale.edu/catalog/16629587.