MLA

Cassirer, Ernst, 1874-1945. Letters of condolence. Box 7, folder 167-170. 1945. https://collections.library.yale.edu/catalog/17289606.

APA, 6th edition

Cassirer, Ernst, 1874-1945. (["Box 7, folder 167-170"])(1945)[Letters of condolence]. https://collections.library.yale.edu/catalog/17289606.