MLA

Wharton, Edith, 1862-1937. Yeats, William Butler. Box 30, folder 947. 1915. https://collections.library.yale.edu/catalog/17111165.

APA, 6th edition

Wharton, Edith, 1862-1937. (["Box 30, folder 947"])(1915)[Yeats, William Butler]. https://collections.library.yale.edu/catalog/17111165.