MLA

Kilpatrick, Jack Frederick (1915-1967). Love letter. Box 2, folder 129. 1939 January 30. https://collections.library.yale.edu/catalog/15492269.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 2, folder 129"])(1939 January 30)[Love letter]. https://collections.library.yale.edu/catalog/15492269.