MLA

Steinberg, Saul, 1914-1999. Chiaromonte, Nicola. Box 12, folder 198. 1958-1969, undated. https://collections.library.yale.edu/catalog/10054206.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 12, folder 198"])(1958-1969, undated)[Chiaromonte, Nicola]. https://collections.library.yale.edu/catalog/10054206.