MLA

Williams, William Carlos, 1883-1963. New Republic. Box 16, folder 512. 1943-1956, undated. https://collections.library.yale.edu/catalog/17049518.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 16, folder 512"])(1943-1956, undated)[New Republic]. https://collections.library.yale.edu/catalog/17049518.