That is made to set teeth on a saw blade. With the slot for the center it will adjust to just about any blade you would normally see. Set every other tooth, turn blade over and set the other teeth. If the anvil (round) part does not have an angle on the edge it could be for straightening the blade or tensioning the blade. Need to find an old saw sharpener or old carpenter that may have used one. Most saw sets that end up here in the what is it ? looking like a pair of pliers.