it is a wire gripper/ for tensioning on strain, ( not joiner) now superseded by gripples (invented after ) which are more versatile and cheaper.What's the idea of them?
got to drill the strainer to pass wire through but only need ordinary wire pullers to pull up not like gripple tool and looks bit tidier if you want to impress on a garden fence or similar.
stronger and more solid do that's why they are more expensive i guess ... similar to the torpedo ones but with a flange