MLA

Williams, William Carlos, 1883-1963. Williams, Paul. Box 25, folder 810. 1926, undated. https://collections.library.yale.edu/catalog/17049564.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 25, folder 810"])(1926, undated)[Williams, Paul]. https://collections.library.yale.edu/catalog/17049564.