MLA

Ford, Julia Ellsworth, 1859-1950. Yeats, Georgie. Box 3, folder 86. 1920 January 29, undated. https://collections.library.yale.edu/catalog/17167844.

APA, 6th edition

Ford, Julia Ellsworth, 1859-1950. (["Box 3, folder 86"])(1920 January 29, undated)[Yeats, Georgie]. https://collections.library.yale.edu/catalog/17167844.