This is the Great Planes ElectriFly 8.0mm to 3/8-24
Prop Adapter with Set Screw and Propeller Nut.

Features

Mounts onto the shaft coming out of the gearbox or motor with the
included set screw. The propeller mounts onto this with the
included nut.
Aluminum prop drive
Metal hardware
One year quality guarantee.
Compatible with any electric flight motor with a 8mm diameter shaft

Includes

One Replacement Prop Drive
One 5/64"X4mm Set Screw
One 9/16 (14mm) Propeller Nut

Requires

Installation onto the motor or gearbox shaft.

Specifications

Length: 1.86" (47.29mm) end to end
1.09" (27.6mm) complete shaft w/.96" (24.4mm) threaded
portion.
Diameter: 3/8" (9.5mm) prop shaft
.31" (8mm) ID for motor shaft
1.4" (35mm) largest part of hub (thrust washer portion)
.62" (15.8mm) smallest part of hub