This is a conical reducer nozzle for the Worx WG521 Turbine 800 leaf blower. It reduces the outlet diameter from 4" down to 2", increasing the velocity of the flow. It slides in from the back of the blower tube, ensuring that it will never come loose during use.
May be compatible with other models so long as the internal diameter at the tube exit is less than 98 mm and the diameter at the inlet (blower side) is > 100 mm.