This is the Great Planes 3mm Prop Adapter. This adapter is used on the Great Planes 540/550 motors.
Split end of adapter goes onto a 3mm prop shaft and compresses as the 5mm nut is tightened in front of the propeller on the other end. This adapter is needed for the slow flyer type props required for electric flight.
Shaft thread and pitch size : 5 x .8mm
For a 5mm domed prop nut, use GPMQ4605. If using this prop adapter with larger size or APC props a spinner would not be able to be used as it not long enough.