MLA

Byron, Robert, 1905-1941. Parsons, Bridget. Box 12, folder 264. 1940, undated. https://collections.library.yale.edu/catalog/16603519.

APA, 6th edition

Byron, Robert, 1905-1941. (["Box 12, folder 264"])(1940, undated)[Parsons, Bridget]. https://collections.library.yale.edu/catalog/16603519.