MLA

Steinberg, Saul, 1914-1999. Notes for designers of postage stamps. Box 60, folder 1099. undated. https://collections.library.yale.edu/catalog/32455020.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 60, folder 1099"])(undated)[Notes for designers of postage stamps]. https://collections.library.yale.edu/catalog/32455020.