MLA

Lewis, Sinclair, 1885-1951. Sauk Centre, Minnesota. Box 56, folder 809-811. 1902, undated. https://collections.library.yale.edu/catalog/2054650.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 56, folder 809-811"])(1902, undated)[Sauk Centre, Minnesota]. https://collections.library.yale.edu/catalog/2054650.