MLA

Williams, William Carlos, 1883-1963. Caetani, Marguerite. Box 3, folder 104. undated. https://collections.library.yale.edu/catalog/11870019.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 3, folder 104"])(undated)[Caetani, Marguerite]. https://collections.library.yale.edu/catalog/11870019.