MLA

Lewis, Sinclair, 1885-1951. Brooks, Van Wyck. Box 46, folder 462. 1944 April 23. https://collections.library.yale.edu/catalog/17116538.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 46, folder 462"])(1944 April 23)[Brooks, Van Wyck]. https://collections.library.yale.edu/catalog/17116538.