MLA

Clésinger-Sand, Solange, 1828-1899. Courtois, Gustav. Box 1, folder 24. undated. https://collections.library.yale.edu/catalog/17444459.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 1, folder 24"])(undated)[Courtois, Gustav]. https://collections.library.yale.edu/catalog/17444459.