MLA

Kilpatrick, Jack Frederick (1915-1967). Letter. Box 12, folder 1086. 1908 December 29. https://collections.library.yale.edu/catalog/15494000.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 12, folder 1086"])(1908 December 29)[Letter]. https://collections.library.yale.edu/catalog/15494000.