MLA

Van Vechten, Carl, 1880-1964. Warfield, William. Box 107, folder 1951. 1951 April 18. https://collections.library.yale.edu/catalog/2028275.

APA, 6th edition

Van Vechten, Carl, 1880-1964. (["Box 107, folder 1951"])(1951 April 18)[Warfield, William]. https://collections.library.yale.edu/catalog/2028275.