MLA

Lewis, Sinclair, 1885-1951. Austin, Mary Hunter. Box 46, folder 447. 1920, undated. https://collections.library.yale.edu/catalog/17116536.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 46, folder 447"])(1920, undated)[Austin, Mary Hunter]. https://collections.library.yale.edu/catalog/17116536.