It's made by putting a heavy lock (e.g.: a master lock) in a balled up sock, then in the other from the pair.
It draws from the Morningstar or "mace and chain" style bludgeoning techniques
Its fast to make, and non-assembled is not considered a deadly weapon. It deals far more damage then merely swinging a heavy chain.
Its sometimes referred to as a smiley due to the mark left by the locks bolt.