MLA

Steinberg, Saul, 1914-1999. Harper & Brothers. Box 21, folder 388-389. 1949-1962, undated. https://collections.library.yale.edu/catalog/10544435.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 21, folder 388-389"])(1949-1962, undated)[Harper & Brothers]. https://collections.library.yale.edu/catalog/10544435.