MLA

Miller, William Winlock, 1822-1876. Powe. Box 4, folder 242. n.d.. https://collections.library.yale.edu/catalog/16700892.

APA, 6th edition

Miller, William Winlock, 1822-1876. (["Box 4, folder 242"])(n.d.)[Powe]. https://collections.library.yale.edu/catalog/16700892.