MLA

Williams, William Carlos, 1883-1963. Edgar Williams. Box 71, folder 1491. 1904-1923. https://collections.library.yale.edu/catalog/2053542.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 71, folder 1491"])(1904-1923)[Edgar Williams]. https://collections.library.yale.edu/catalog/2053542.