This brand new Yuil padlock, measuring 75x37x14cms, is beautifully engineered.  It reminds me of the Chubb Escort padlock as the key which opens it is only released if and when the padlock is closed locked.  The internal lock will also work using a standard handcuff key but trying this is not recommended as non Yuil keys might jam - you have been warned! These can be used in conjunction with the Black Box system to hold the chain in place on the outside of the Black box. Just search Black Box.