MLA

Kilpatrick, Jack Frederick (1915-1967). Letter, unidentified. Box 7, folder 560. undated. https://collections.library.yale.edu/catalog/15492785.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 7, folder 560"])(undated)[Letter, unidentified]. https://collections.library.yale.edu/catalog/15492785.