MLA

Williams, William Carlos, 1883-1963. Dartmouth College. Box 5, folder 156. 1939. https://collections.library.yale.edu/catalog/17049467.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 5, folder 156"])(1939)[Dartmouth College]. https://collections.library.yale.edu/catalog/17049467.