MLA

Williams, William Carlos, 1883-1963. Burke, Lib. Box 3, folder 100. 1955-1960, undated. https://collections.library.yale.edu/catalog/11870018.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 3, folder 100"])(1955-1960, undated)[Burke, Lib]. https://collections.library.yale.edu/catalog/11870018.