MLA

Whitman, Walt, 1819-1892. [Gold?] coin. Box 4, folder 201. n.d.. https://collections.library.yale.edu/catalog/2012036.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 4, folder 201"])(n.d.)[[Gold?] coin]. https://collections.library.yale.edu/catalog/2012036.