MLA

Williams, William Carlos, 1883-1963. Oppen, George. Box 17, folder 547. 1962, undated. https://collections.library.yale.edu/catalog/16051907.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 17, folder 547"])(1962, undated)[Oppen, George]. https://collections.library.yale.edu/catalog/16051907.