The Marietta College Library (Marietta, Ohio), can be searched for Manuscripts and Documents of the Ohio Company of Associates here. A search for "Backus" produced a Jonas Prentice letter to James Backus (description below):
"Jonas Prentice sent a copy of this printed letter to James Backus, who held a share in the Ohio Company, in the late Samuel H. Parsons' (now Prentice's) agency. Printed with the letter is a list of proprietors in Parsons' agency and a letter from Rufus Putnam in response to a copy of Prentice's letter."
A genealogical memoir of James Backus can be explored here.
Also see James Backus surveyors field notes at the Marietta College site (and an earlier post on In Deeds).