MLA

Byron, Robert, 1905-1941. Unidentified. Box 53, folder 772. undated. https://collections.library.yale.edu/catalog/2034215.

APA, 6th edition

Byron, Robert, 1905-1941. (["Box 53, folder 772"])(undated)[Unidentified]. https://collections.library.yale.edu/catalog/2034215.