If it is rusty, it has to be old, thus it is Civil War related, huh?
If you looked closely, using the zoom on the page for the first picture, there appears to be a stamped marking just below the bigger end, on the shank.
Doubt very seriously it that old, looks like a stamped or drop forged wrench to me. Note the smaller square opening has a relieved area around the opening on one side. That would have to be either forged in, or machined in.
My guess it is for some type of torch, or another WAG would be maybe some type of spray gun apparatus.