MLA

Clésinger-Sand, Solange, 1828-1899. Choiecki, [Charles] Edward. Box 1, folder 21. undated. https://collections.library.yale.edu/catalog/16620193.

APA, 6th edition

Clésinger-Sand, Solange, 1828-1899. (["Box 1, folder 21"])(undated)[Choiecki, [Charles] Edward]. https://collections.library.yale.edu/catalog/16620193.