Most wire stretchers that I have seen have some type of lever or pully on it to help increase the tension. But, I guess that just plain brute force would do as well. Nice site with some old fence stretchers is at: http://www.jacobhaishmfg.org/Fencing%20Tools.htm
There was a Townsend that made antique fence tools - but no pics are available. Just has had items at auctions.
What lends you to believe it is a fence tool?