MLA

Bird, William, 1888-1963. Canto IV. Box 2, folder 72. n.d.. https://collections.library.yale.edu/catalog/16164962.

APA, 6th edition

Bird, William, 1888-1963. (["Box 2, folder 72"])(n.d.)[Canto IV]. https://collections.library.yale.edu/catalog/16164962.