MLA

Backus, Isaac, 1724-1806. volume 9. Box 1, folder 7. 1776 January 7–1780 July 27. https://collections.library.yale.edu/catalog/33297942.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 1, folder 7"])(1776 January 7–1780 July 27)[volume 9]. https://collections.library.yale.edu/catalog/33297942.