MLA

Clésinger-Sand, Solange, 1828-1899. Nencioni, Enrico. Box 1, folder 45. undated. https://collections.library.yale.edu/catalog/17444478.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 1, folder 45"])(undated)[Nencioni, Enrico]. https://collections.library.yale.edu/catalog/17444478.