MLA

Steinberg, Saul, 1914-1999. Merry Christmas. Box 76, folder 1478. circa 1944. https://collections.library.yale.edu/catalog/16855976.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 76, folder 1478"])(circa 1944)[Merry Christmas]. https://collections.library.yale.edu/catalog/16855976.