Plastic pulley wheel, designed to slip onto a karabiner to make a pulley.
Its main advantages are low cost and low weight.
Its main disadvantage is that it doesn't fit on many krabs, and only works really well on Petzl oval shaped ones - in fact Petzl make a special krabs - the OK Oval Screwgate and the OWall Oval Plaingate - just for it.
Working load: 1 kN.
Weight: 10g.