MLA

Whitman, Walt, 1819-1892. Postcard. Box 4, folder 202. n.d.. https://collections.library.yale.edu/catalog/2053831.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 4, folder 202"])(n.d.)[Postcard]. https://collections.library.yale.edu/catalog/2053831.