MLA

Berberova, N. (Nina), 1901-1993. Sieveking, Edda. Box 54, folder 1248. n.d.. https://collections.library.yale.edu/catalog/10454475.

APA, 6th edition

Berberova, N. (Nina), 1901-1993. (["Box 54, folder 1248"])(n.d.)[Sieveking, Edda]. https://collections.library.yale.edu/catalog/10454475.