i seem to remember that the older GM's had a system where the was a 'hollow' locking screw with a smaller hole in the middle through which you could get to the main adjustment bolt. In that case you had to use a larger allen key to unlock the mechanism a bit, then a smaller one that you threaded through the hole in the top screw to adjust the weight.
On my one which was in the last generation to be made, there was a grub screw with the same thread as the mechanism that had to be removed completely (don't lose it whatever you do!!!) and then you alter the 'little box's' position by turning the second screw that was hiding beneath the first. The limb bolt was actually threaded which meant that it slowly moved as you turned the screw, *BUT* also meant that, on mine, the thread could strip, allowing it to slowly grind back to the lowest setting whilst you were shooting!
