MLA

Williams, William Carlos, 1883-1963. The Poetry Center. Box 18, folder 587. 1954-1960. https://collections.library.yale.edu/catalog/17049530.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 18, folder 587"])(1954-1960)[The Poetry Center]. https://collections.library.yale.edu/catalog/17049530.