MLA

Backus, Isaac, 1724-1806. Civil records: Massachusetts. Box 6, folder 1. Undated. https://collections.library.yale.edu/catalog/33553783.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 6, folder 1"])(Undated)[Civil records: Massachusetts]. https://collections.library.yale.edu/catalog/33553783.