MLA

Williams, William Carlos, 1883-1963. Niedecker, Lorine. Box 16, folder 526. undated. https://collections.library.yale.edu/catalog/17049520.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 16, folder 526"])(undated)[Niedecker, Lorine]. https://collections.library.yale.edu/catalog/17049520.