MLA

Fitzgerald, Robert, 1910-1985. Williams, William Carlos. Box 40, folder 1587. 1949 May 3. https://collections.library.yale.edu/catalog/17093657.

APA, 6th edition

Fitzgerald, Robert, 1910-1985. (["Box 40, folder 1587"])(1949 May 3)[Williams, William Carlos]. https://collections.library.yale.edu/catalog/17093657.