MLA

Anderson, Margaret C., 1886-1973. Abbott, Berenice. Box 18, folder 326. undated. https://collections.library.yale.edu/catalog/2020078.

APA, 6th edition

Anderson, Margaret C., 1886-1973. (["Box 18, folder 326"])(undated)[Abbott, Berenice]. https://collections.library.yale.edu/catalog/2020078.