MLA

Whitman, Walt, 1819-1892. Unidentified. Box 10, folder 212. 1879 Apr 13. https://collections.library.yale.edu/catalog/2053841.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 10, folder 212"])(1879 Apr 13)[Unidentified]. https://collections.library.yale.edu/catalog/2053841.