MLA

Cassirer, Ernst, 1874-1945. Hönigswald, Richard. Box 2, folder 51. 1937 Apr 8. https://collections.library.yale.edu/catalog/17289605.

APA, 6th edition

Cassirer, Ernst, 1874-1945. (["Box 2, folder 51"])(1937 Apr 8)[Hönigswald, Richard]. https://collections.library.yale.edu/catalog/17289605.