MLA

Kilpatrick, Jack Frederick (1915-1967). Rattler, John. Letter. Box 17, folder 1442. 1927 March 31. https://collections.library.yale.edu/catalog/15532146.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 17, folder 1442"])(1927 March 31)[Rattler, John. Letter]. https://collections.library.yale.edu/catalog/15532146.