MLA

Walpole, Horace, 1717-1797. Letter from Horace Walpole. 1, folder 1a. Circa 1766 Sep 26. https://collections.library.yale.edu/catalog/16227521.

APA, 6th edition

Walpole, Horace, 1717-1797. (["1, folder 1a"])(Circa 1766 Sep 26)[Letter from Horace Walpole]. https://collections.library.yale.edu/catalog/16227521.