MLA

Whitman, Walt, 1819-1892. Unidentified. Box 1, folder 21. 1887, n.d.. https://collections.library.yale.edu/catalog/2053655.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 21"])(1887, n.d.)[Unidentified]. https://collections.library.yale.edu/catalog/2053655.