MLA

Lewis, Sinclair, 1885-1951. West, Rebecca. Box 50, folder 646. 1921-1948. https://collections.library.yale.edu/catalog/17116556.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 50, folder 646"])(1921-1948)[West, Rebecca]. https://collections.library.yale.edu/catalog/17116556.