MLA

Steinberg, Saul, 1914-1999. Untitled [Envelope with faux calligraphy address and faux stamps and postal cancellation]. Box 76, folder 1489. undated. https://collections.library.yale.edu/catalog/16855987.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 76, folder 1489"])(undated)[Untitled [Envelope with faux calligraphy address and faux stamps and postal cancellation]]. https://collections.library.yale.edu/catalog/16855987.