MLA

Johnson, James Weldon, 1871-1938. Spencer, Anne. Box 19, folder 449. undated. https://collections.library.yale.edu/catalog/16748009.

APA, 6th edition

Johnson, James Weldon, 1871-1938. (["Box 19, folder 449"])(undated)[Spencer, Anne]. https://collections.library.yale.edu/catalog/16748009.