MLA

Anderson, Margaret C., 1886-1973. Flanner, Janet. Box 20, folder 357. 1965, undated. https://collections.library.yale.edu/catalog/2035265.

APA, 6th edition

Anderson, Margaret C., 1886-1973. (["Box 20, folder 357"])(1965, undated)[Flanner, Janet]. https://collections.library.yale.edu/catalog/2035265.