MLA

Kilpatrick, Jack Frederick (1915-1967). Sawny, Jennie. Letter. Box 7, folder 575. undated. https://collections.library.yale.edu/catalog/15492875.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 7, folder 575"])(undated)[Sawny, Jennie. Letter]. https://collections.library.yale.edu/catalog/15492875.