MLA

Nicholls, Norton, 1741?-1809. -- Kunigl to Norton Nicholls. Milan. Concerns Count Wilseck; Count Charles Firmian. Box 2, folder 199. 1773 Jul 4. https://collections.library.yale.edu/catalog/17439996.

APA, 6th edition

Nicholls, Norton, 1741?-1809. (["Box 2, folder 199"])(1773 Jul 4)[-- Kunigl to Norton Nicholls. Milan. Concerns Count Wilseck; Count Charles Firmian]. https://collections.library.yale.edu/catalog/17439996.