MLA

Stepun, Fedor, 1884-1965. Windelband, Gisela. Box 37, folder 1304. 1964 Feb 20. https://collections.library.yale.edu/catalog/16495065.

APA, 6th edition

Stepun, Fedor, 1884-1965. (["Box 37, folder 1304"])(1964 Feb 20)[Windelband, Gisela]. https://collections.library.yale.edu/catalog/16495065.