MLA

Van Vechten, Carl, 1880-1964. McKay, Claude. Box 223. 1941 July 25. https://collections.library.yale.edu/catalog/2019351.

APA, 6th edition

Van Vechten, Carl, 1880-1964. (["Box 223"])(1941 July 25)[McKay, Claude]. https://collections.library.yale.edu/catalog/2019351.