MLA

Coffin, William Sloane, Jr., 1924-2006. Law and Order and Youth: An Open Letter to Hubert Humphrey and Richard Nixon. Box 42, folder 100. 1968. https://collections.library.yale.edu/catalog/10080186.

APA, 6th edition

Coffin, William Sloane, Jr., 1924-2006. (["Box 42, folder 100"])(1968)[Law and Order and Youth: An Open Letter to Hubert Humphrey and Richard Nixon]. https://collections.library.yale.edu/catalog/10080186.