MLA

Van Sinderen, Alfred White, 1924-1998. Abraham Lincoln to P.S. Harrison, Tremont, Illinois. Box 1, folder 31. 1849 September 23. https://collections.library.yale.edu/catalog/10016849.

APA, 6th edition

Van Sinderen, Alfred White, 1924-1998. (["Box 1, folder 31"])(1849 September 23)[Abraham Lincoln to P.S. Harrison, Tremont, Illinois]. https://collections.library.yale.edu/catalog/10016849.