This is a Great Planes ElectriFly 4.0mm - 1/4-28 Prop Drive (adapter)
with Set Screw and Prop Nut.

Features

Mounts onto the shaft coming out of a 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 4mm motor shaft with a 1/4" prop shaft.

Includes

One Prop Drive
One 3.5x5mm Set Screw
One 7/16 Propeller Nut

Requires

Installation onto the motor or gearbox shaft.

Specifications

Length: 1.5" (37mm) end to end
.98" (25mm) complete shaft w/.69" (17.6mm) threaded
portion.
Diameter: 1/46" (6.27mm) prop shaft
.16" (4mm) ID for motor shaft
.78" (20mm) largest part of hub (thrust washer portion)
.47" (12mm) smallest part of hub