MLA

Backus, Isaac, 1724-1806. volume 6. Box 1, folder 4. 1763 July 20–1767 October 11. https://collections.library.yale.edu/catalog/33297936.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 1, folder 4"])(1763 July 20–1767 October 11)[volume 6]. https://collections.library.yale.edu/catalog/33297936.