MLA

Lewis, Sinclair, 1885-1951. Salvemini, Gaetano. Box 49, folder 602. 1936 April 13. https://collections.library.yale.edu/catalog/17116551.

APA, 6th edition

Lewis, Sinclair, 1885-1951. (["Box 49, folder 602"])(1936 April 13)[Salvemini, Gaetano]. https://collections.library.yale.edu/catalog/17116551.