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

Includes

One prop adapter
One prop washer
One 5/16-24 prop nut

Requires

Tightening the prop nut to compress the prop adapter onto the motor
shaft

Specifications

Total length: 1.57" (40mm)
Diameter of prop washer: 1.1" (28mm)