MLA

Whitman, Walt, 1819-1892. Oldach, ----. Box 10, folder 210. 1890 Oct 21. https://collections.library.yale.edu/catalog/2053839.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 10, folder 210"])(1890 Oct 21)[Oldach, ----]. https://collections.library.yale.edu/catalog/2053839.