MLA

Whitman, Walt, 1819-1892. "Testimonial to Walt Whitman," page proofs. Box 3, folder 174. [1890]. https://collections.library.yale.edu/catalog/32455825.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 3, folder 174"])([1890])["Testimonial to Walt Whitman," page proofs]. https://collections.library.yale.edu/catalog/32455825.