MLA

Nicholls, Norton, 1741?-1809. Norton Nicholls to Jane Floyer Nicholls. Bern. Concerns Thomas Gray; Horace Walpole. Box 1, folder 120. 1771 Sep 5. https://collections.library.yale.edu/catalog/17439917.

APA, 6th edition

Nicholls, Norton, 1741?-1809. (["Box 1, folder 120"])(1771 Sep 5)[Norton Nicholls to Jane Floyer Nicholls. Bern. Concerns Thomas Gray; Horace Walpole]. https://collections.library.yale.edu/catalog/17439917.