MLA

Kilpatrick, Jack Frederick (1915-1967). Belvin, B. Frank. Typed letter in English to "Fellow-worker" about Christian literature. Box 22, folder 1774. 1947 May 15. https://collections.library.yale.edu/catalog/15532578.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 22, folder 1774"])(1947 May 15)[Belvin, B. Frank. Typed letter in English to "Fellow-worker" about Christian literature]. https://collections.library.yale.edu/catalog/15532578.