MLA

Whitman, Walt, 1819-1892. [Kurtz, William?]. Box 11, folder 229. [1892?]. https://collections.library.yale.edu/catalog/2053854.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 11, folder 229"])([1892?])[[Kurtz, William?]]. https://collections.library.yale.edu/catalog/2053854.