MLA

Lewis, Sinclair, 1885-1951. Engle, Paul. Box 46, folder 488. 1944-1945. https://collections.library.yale.edu/catalog/17116540.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 46, folder 488"])(1944-1945)[Engle, Paul]. https://collections.library.yale.edu/catalog/17116540.