MLA

Whitman, Walt, 1819-1892. G[ustav?] Kruell engraving. Box 11, folder 235. n.d.. https://collections.library.yale.edu/catalog/2054634.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 11, folder 235"])(n.d.)[G[ustav?] Kruell engraving]. https://collections.library.yale.edu/catalog/2054634.