MLA

Williams, William Carlos, 1883-1963. Rutgers University. Box 21, folder 648. 1949-1958, undated. https://collections.library.yale.edu/catalog/17049540.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 21, folder 648"])(1949-1958, undated)[Rutgers University]. https://collections.library.yale.edu/catalog/17049540.