MLA

Steinberg, Saul, 1914-1999. Untitled [New Yorker envelope addressed to Saul Steinberg with faux stamps and cancellations]. Box 76, folder 1488. undated. https://collections.library.yale.edu/catalog/16855986.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 76, folder 1488"])(undated)[Untitled [New Yorker envelope addressed to Saul Steinberg with faux stamps and cancellations]]. https://collections.library.yale.edu/catalog/16855986.