MLA

Williams, William Carlos, 1883-1963. New Leader. Box 16, folder 509. 1939. https://collections.library.yale.edu/catalog/17049516.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 16, folder 509"])(1939)[New Leader]. https://collections.library.yale.edu/catalog/17049516.