MLA

Backus, Isaac, 1724-1806. volume 8. Box 1, folder 6. 1771 October 25–1775 December 31. https://collections.library.yale.edu/catalog/33297940.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 1, folder 6"])(1771 October 25–1775 December 31)[volume 8]. https://collections.library.yale.edu/catalog/33297940.