MLA

Dreier, Katherine S. (Katherine Sophie), 1877-1952. Schwitters, Kurt to Herr Baumeister. Box 41, folder 1209. 1927 Jan 28. https://collections.library.yale.edu/catalog/16694434.

APA, 6th edition

Dreier, Katherine S. (Katherine Sophie), 1877-1952. (["Box 41, folder 1209"])(1927 Jan 28)[Schwitters, Kurt to Herr Baumeister]. https://collections.library.yale.edu/catalog/16694434.