MLA

Kilpatrick, Jack Frederick (1915-1967). Letter. Box 3, folder 224. undated. https://collections.library.yale.edu/catalog/15492329.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 3, folder 224"])(undated)[Letter]. https://collections.library.yale.edu/catalog/15492329.