MLA

Kilpatrick, Jack Frederick (1915-1967). Sawney, Alex. Letter. Box 8, folder 725. 1921 December 17. https://collections.library.yale.edu/catalog/15493050.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 8, folder 725"])(1921 December 17)[Sawney, Alex. Letter]. https://collections.library.yale.edu/catalog/15493050.