MLA

Williams, William Carlos, 1883-1963. Anderson, Margaret. Box 1, folder 19. 1951-1953. https://collections.library.yale.edu/catalog/17049442.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 1, folder 19"])(1951-1953)[Anderson, Margaret]. https://collections.library.yale.edu/catalog/17049442.