MLA

Kilpatrick, Jack Frederick (1915-1967). Business letter. Box 7, folder 559. undated. https://collections.library.yale.edu/catalog/15492784.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 7, folder 559"])(undated)[Business letter]. https://collections.library.yale.edu/catalog/15492784.