MLA

Prokosch, Frederic, 1908-1989. Prokosch, Frederic. Box 1 | Folder 21. [1940] November 20. https://collections.library.yale.edu/catalog/17104151.

APA, 6th edition

Prokosch, Frederic, 1908-1989. (["Box 1 | Folder 21"])([1940] November 20)[Prokosch, Frederic]. https://collections.library.yale.edu/catalog/17104151.