MLA

Warren, Robert Penn, 1905-1989. William Morris Agency. Box 78, folder 1541-48. 1940-55. https://collections.library.yale.edu/catalog/2007733.

APA, 6th edition

Warren, Robert Penn, 1905-1989. (["Box 78, folder 1541-48"])(1940-55)[William Morris Agency]. https://collections.library.yale.edu/catalog/2007733.