MLA

Whitman, Walt, 1819-1892. Flood, John. Box 1, folder 27. 1869 [Jan?] 11. https://collections.library.yale.edu/catalog/2053661.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 27"])(1869 [Jan?] 11)[Flood, John]. https://collections.library.yale.edu/catalog/2053661.