MLA

Steinberg, Saul, 1914-1999. Trilling, Diana. Box 49, folder 885. 1965 December 4. https://collections.library.yale.edu/catalog/10034217.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 49, folder 885"])(1965 December 4)[Trilling, Diana]. https://collections.library.yale.edu/catalog/10034217.