MLA

Williams, William Carlos, 1883-1963. Horizon. Box 8, folder 288. 1959. https://collections.library.yale.edu/catalog/17049489.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 8, folder 288"])(1959)[Horizon]. https://collections.library.yale.edu/catalog/17049489.