This is the Great Planes ElectriFly 6mm Prop Adapter Motor Shaft Collet
5/16-24 Output Shaft.
Prop Adapter to Fit Standard Size Propellers onto a 6mm Motor Shaft.

Features

Large shaft size for Brushless (or Brushed) motors with smooth
(non-threaded) 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: 1.72" (44mm)
Diameter of drive washer and prop washer: 1.2" (30mm)