MLA

Stein, Gertrude, 1874-1946. Keyser, Ephraim. Box 153, folder 3770-3772. n.d.. https://collections.library.yale.edu/catalog/9998174.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 153, folder 3770-3772"])(n.d.)[Keyser, Ephraim]. https://collections.library.yale.edu/catalog/9998174.