MLA

Whitman, Walt, 1819-1892. Galley proof. Box 3, folder 150. n.d.. https://collections.library.yale.edu/catalog/2053782.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 3, folder 150"])(n.d.)[Galley proof]. https://collections.library.yale.edu/catalog/2053782.