MLA

Lewis, Sinclair, 1885-1951. Lindsay, Vachel. Box 48, folder 554. 1920 December 15. https://collections.library.yale.edu/catalog/17116545.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 48, folder 554"])(1920 December 15)[Lindsay, Vachel]. https://collections.library.yale.edu/catalog/17116545.