MLA

Thoreau, Henry David, 1817-1862. Holograph. Box 1, folder 11. [1851 Aug?]. https://collections.library.yale.edu/catalog/2064509.

APA, 6th edition

Thoreau, Henry David, 1817-1862. (["Box 1, folder 11"])([1851 Aug?])[Holograph]. https://collections.library.yale.edu/catalog/2064509.