MLA

Ripley, Sidney Dillon, 1913-2001, Smithsonian Institution. [Letter : [late 1966?], Washington (D.C.), Smithsonian Institution to Saul Steinberg, New York]. Box 16 | Folder: Correspondence 1966-67 [3]. undated. https://collections.library.yale.edu/catalog/10054812.

APA, 6th edition

Ripley, Sidney Dillon, 1913-2001, & Smithsonian Institution. (["Box 16 | Folder: Correspondence 1966-67 [3]"])(undated)[[Letter : [late 1966?], Washington (D.C.), Smithsonian Institution to Saul Steinberg, New York]]. https://collections.library.yale.edu/catalog/10054812.