MLA

Steinberg, Saul, 1914-1999. Sigrid Spaeth tombstone. Box 249, folder 4260. undated. https://collections.library.yale.edu/catalog/10189947.

APA, 6th edition

Steinberg, Saul, 1914-1999. (["Box 249, folder 4260"])(undated)[Sigrid Spaeth tombstone]. https://collections.library.yale.edu/catalog/10189947.