MLA

Backus, Isaac, 1724-1806. volume 4. Box 1, folder 3. 1755 April 20–1759 June 28. https://collections.library.yale.edu/catalog/33297935.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 1, folder 3"])(1755 April 20–1759 June 28)[volume 4]. https://collections.library.yale.edu/catalog/33297935.