MLA

Butts, Mary, 1890-1937. Cocteau, Jean. Box 1, folder 15. 1926-29, n.d.. https://collections.library.yale.edu/catalog/16893642.

APA, 6th edition

Butts, Mary, 1890-1937. (["Box 1, folder 15"])(1926-29, n.d.)[Cocteau, Jean]. https://collections.library.yale.edu/catalog/16893642.