MLA

Kilpatrick, Jack Frederick (1915-1967). Kingfisher, Pauline. Letter. Box 7, folder 576. undated. https://collections.library.yale.edu/catalog/15492876.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 7, folder 576"])(undated)[Kingfisher, Pauline. Letter]. https://collections.library.yale.edu/catalog/15492876.