MLA

Steinberg, Saul, 1914-1999. Untitled [Self-addressed, stamped envelope with postmark and faux stamp with faux letter inside]. Box 76, folder 1437. 1980. https://collections.library.yale.edu/catalog/16855936.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 76, folder 1437"])(1980)[Untitled [Self-addressed, stamped envelope with postmark and faux stamp with faux letter inside]]. https://collections.library.yale.edu/catalog/16855936.