MLA

Whitman, Walt, 1819-1892. Burroughs, John. Box 2, folder 61. [1873?]-88. https://collections.library.yale.edu/catalog/2053695.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 61"])([1873?]-88)[Burroughs, John]. https://collections.library.yale.edu/catalog/2053695.