MLA

Butts, Mary, 1890-1937. Pound, Ezra. Box 1, folder 33. 1931 Dec 2, n.d.. https://collections.library.yale.edu/catalog/16893660.

APA, 6th edition

Butts, Mary, 1890-1937. (["Box 1, folder 33"])(1931 Dec 2, n.d.)[Pound, Ezra]. https://collections.library.yale.edu/catalog/16893660.