This is the Great Planes ElectriFly 5mm to 5/16-24 prop adapter with collet and propeller nut.
Features: Large shaft size for Brushless or Brushed motors with 5mm shafts
Requires: Tightening the prop nut to compress the prop adapter onto the motor shaft
Total length: 1.57 (40mm)
Diameter of prop washer: 1.1 (28mm)