MLA

Kilpatrick, Jack Frederick (1915-1967). Letter. Box 12, folder 1085. 1891 January 4. https://collections.library.yale.edu/catalog/15493999.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 12, folder 1085"])(1891 January 4)[Letter]. https://collections.library.yale.edu/catalog/15493999.