MLA

Whitman, Walt, 1819-1892. Empty envelopes. Box 2, folder 109. 1921 Dec 21. https://collections.library.yale.edu/catalog/2053743.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 109"])(1921 Dec 21)[Empty envelopes]. https://collections.library.yale.edu/catalog/2053743.