MLA

White, Walter, 1893-1955. Selznick, David O.. Box 6, folder 190. 1946 April 16. https://collections.library.yale.edu/catalog/16745946.

APA, 6th edition

White, Walter, 1893-1955. (["Box 6, folder 190"])(1946 April 16)[Selznick, David O.]. https://collections.library.yale.edu/catalog/16745946.