MLA

Acton, Harold, 1904-1994. Correspondence. Box 3, folder 168. 1939-1940. https://collections.library.yale.edu/catalog/16604599.

APA, 6th edition

Acton, Harold, 1904-1994. (["Box 3, folder 168"])(1939-1940)[Correspondence]. https://collections.library.yale.edu/catalog/16604599.