MLA

Dreier, Katherine S. (Katherine Sophie), 1877-1952. Heythum, Antonin; Heythum, Charlotta. Box 17, folder 489. 1946-1947, undated. https://collections.library.yale.edu/catalog/17047945.

APA, 6th edition

Dreier, Katherine S. (Katherine Sophie), 1877-1952. (["Box 17, folder 489"])(1946-1947, undated)[Heythum, Antonin; Heythum, Charlotta]. https://collections.library.yale.edu/catalog/17047945.