MLA

Berberova, N. (Nina), 1901-1993. Gorbanevskaia, Natasha. Box 9, folder 209. 1981 Dec 15. https://collections.library.yale.edu/catalog/16687999.

APA, 6th edition

Berberova, N. (Nina), 1901-1993. (["Box 9, folder 209"])(1981 Dec 15)[Gorbanevskaia, Natasha]. https://collections.library.yale.edu/catalog/16687999.