This is a Great Planes ElectriFly 3.175mm - 5mm Prop Drive (adapter)
with Set Screw and Prop Nut.

Features

Mounts onto the 3.175mm shaft coming out of a gearbox or motor
shaft using a set screw. The propeller mounts onto this with the
included nut.
Aluminum prop drive
Metal hardware
One year quality guarantee

Includes

One Prop Drive
One 1.5mmx3mm set screw
One 5/16 Propeller Nut

Specifications

Length: 1.1" (28mm) end to end
.71" (18mm) shaft w/.54" (14mm) threaded portion
Diameter: 5mm prop shaft
3.175mm ID for motor shaft
19mm largest part of hub (thrust washer portion)
12.7mm smallest part of hub