MLA

Fitts, Dudley, 1903-1968. MacLeish, Archibald. Box 1, folder 40. 1941-1942. https://collections.library.yale.edu/catalog/17122508.

APA, 6th edition

Fitts, Dudley, 1903-1968. (["Box 1, folder 40"])(1941-1942)[MacLeish, Archibald]. https://collections.library.yale.edu/catalog/17122508.