MLA

Williams, William Carlos, 1883-1963. McCullers, Carson. Box 14, folder 434. undated. https://collections.library.yale.edu/catalog/17049507.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 14, folder 434"])(undated)[McCullers, Carson]. https://collections.library.yale.edu/catalog/17049507.