MLA

Steinberg, Saul, 1914-1999. Ashton, Dore. Box 4, folder 50. 1956-1996 undated. https://collections.library.yale.edu/catalog/10000043.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 4, folder 50"])(1956-1996 undated)[Ashton, Dore]. https://collections.library.yale.edu/catalog/10000043.