MLA

Clésinger-Sand, Solange, 1828-1899. Poncy, Charles. Box 2, folder 48-49. 1865-1880, undated. https://collections.library.yale.edu/catalog/17444481.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 2, folder 48-49"])(1865-1880, undated)[Poncy, Charles]. https://collections.library.yale.edu/catalog/17444481.