MLA

Hughes, Langston, 1902-1967. Cather, Willa. Box 43, folder 749. 1941 Apr 15. https://collections.library.yale.edu/catalog/16708777.

APA, 6th edition

Hughes, Langston, 1902-1967. (["Box 43, folder 749"])(1941 Apr 15)[Cather, Willa]. https://collections.library.yale.edu/catalog/16708777.