MLA

Berberova, N. (Nina), 1901-1993. Sylvester, Richard. Box 20, folder 546-550. 1973-87, n.d.. https://collections.library.yale.edu/catalog/32325290.

APA, 6th edition

Berberova, N. (Nina), 1901-1993. (["Box 20, folder 546-550"])(1973-87, n.d.)[Sylvester, Richard]. https://collections.library.yale.edu/catalog/32325290.