In reply to Kevster:
The superlight rocks have a single wire cable, silver soldered into the nut. They do a small silver, and a gold size that are not present in the superlight offset rocks. The rest of the superlight rocks are no better than the superlight offsets IMO (and are potentially worse - I don't use the rest).
So the gold and the silver little ones, have an offset side, and a 'half-normal nut' (really narrow 'rock curve') side. This half-width lets them act really well as 'keys' - where you put it into a horizontal slot, and rotate 90 degrees and it gets wedged in good (crucially it has a rectangular profile at this point - there is no left/right taper - this gives it really good stability). Normal offsets (the offset shape means that it can be difficult or not possible to find a good constriction in a horizontal crack) or normal don't have this ability exactly the same (to 'key' in a normal nut using a 90 degree turn requires a massive hole, and then a much smaller opening to the side, unlikely and potentially not as safe), and so can't be used in nearly-uniform horizontal cracks.
Secondly they have the ability to sit into normal small-gold and small-silver placements, as well as being used as small offsets. For this reason, and the unique 'keying' ability mentioned above - they are extremely useful as a second set of nuts.