MLA

Kilpatrick, Jack Frederick (1915-1967). Acorn, John. Letter. Box 13, folder 1111. 1916 June 30. https://collections.library.yale.edu/catalog/15504934.

APA, 6th edition

Kilpatrick, Jack Frederick (1915-1967). (["Box 13, folder 1111"])(1916 June 30)[Acorn, John. Letter]. https://collections.library.yale.edu/catalog/15504934.