MLA

Nicholls, Norton, 1741?-1809. Charles de Bonstetten to Norton Nicholls. Milan. Concerns Kunigl. Box 2, folder 212. 1773 Dec 20. https://collections.library.yale.edu/catalog/17440009.

APA, 6th edition

Nicholls, Norton, 1741?-1809. (["Box 2, folder 212"])(1773 Dec 20)[Charles de Bonstetten to Norton Nicholls. Milan. Concerns Kunigl]. https://collections.library.yale.edu/catalog/17440009.