MLA

Whitman, Walt, 1819-1892. Whitman, Thomas Jefferson. Box 2, folder 91. 1873 Mar 16. https://collections.library.yale.edu/catalog/2053725.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 91"])(1873 Mar 16)[Whitman, Thomas Jefferson]. https://collections.library.yale.edu/catalog/2053725.