MLA

Steinberg, Saul, 1914-1999. Shawn, William. Box 43, folder 780. 1959-1992, undated. https://collections.library.yale.edu/catalog/10545532.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 43, folder 780"])(1959-1992, undated)[Shawn, William]. https://collections.library.yale.edu/catalog/10545532.