MLA

Berberova, N. (Nina), 1901-1993. Bely, Andrey. Box 54, folder 1226. n.d.. https://collections.library.yale.edu/catalog/10454437.

APA, 6th edition

Berberova, N. (Nina), 1901-1993. (["Box 54, folder 1226"])(n.d.)[Bely, Andrey]. https://collections.library.yale.edu/catalog/10454437.