MLA

Lewis, Sinclair, 1885-1951. Leonard, George. Box 51, folder 678. 1944 October 25. https://collections.library.yale.edu/catalog/17116559.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 51, folder 678"])(1944 October 25)[Leonard, George]. https://collections.library.yale.edu/catalog/17116559.