This key was a 2nd attempt at using a spring torsion bar which is held in the front of the key and to which the main arm is brazed on, by twisting the torsion spring ends and locking them it would set the tension of the key, this proved to be a problem as although once set it was fine the adjusting process was difficult, much later I fitted a soft coil spring to offset the hard tension spring  and  give an easy adjustment of the tension, this has make it into a fine little key which I've used a lot