MLA

Dreier, Katherine S. (Katherine Sophie), 1877-1952. Schwitters, Kurt. Box 31, folder 925-926. 1925-47, n.d.. https://collections.library.yale.edu/catalog/2042595.

APA, 6th edition

Dreier, Katherine S. (Katherine Sophie), 1877-1952. (["Box 31, folder 925-926"])(1925-47, n.d.)[Schwitters, Kurt]. https://collections.library.yale.edu/catalog/2042595.