This is the Great Planes ElectriFly 3.0mm - 5mm
Prop Adapter with Set Screw and Propeller Nut.

Features

Compatible with any electric flight motor having a 3mm diameter shaft
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
Fits a 3mm motor shaft with a 5mm prop shaft

Includes

One Prop Adapter
One 3.5x5mm Set Screw
One 5/16 Propeller Nut

Requires

Installation onto the motor or gearbox shaft

Specifications

Length: 1.1" (28.9mm) end to end
.69" (17.6mm) complete shaft w/.45" (11.3mm) threaded
portion.
Diameter: 3/16" (4.76mm) prop shaft
.12" (3mm) ID for motor shaft
.66" (17mm) largest part of hub (thrust washer portion)
.43" (11mm) smallest part of hub