MLA

Ripley, Sidney Dillon, 1913-2001, Smithsonian Institution. [Letter : 1967 March 6, Washington (D.C.), Smithsonian Institution to Saul Steinberg, Washington (D.C.)]. Box 16 | Folder: Correspondence 1967 [3]. 1967 March 6. https://collections.library.yale.edu/catalog/10058500.

APA, 6th edition

Ripley, Sidney Dillon, 1913-2001, & Smithsonian Institution. (["Box 16 | Folder: Correspondence 1967 [3]"])(1967 March 6)[[Letter : 1967 March 6, Washington (D.C.), Smithsonian Institution to Saul Steinberg, Washington (D.C.)]]. https://collections.library.yale.edu/catalog/10058500.