This is a Great Planes ElectriFly 3.175mm - 5mm prop drive (adapter) with set screw and prop nut.
End to end length: 1.1 (28mm)
Shaft length: 0.71 (18mm) with 0.54 (14mm) threaded portion
Prop shaft diameter: 5mm
ID for motor shaft diameter: 3.175mm
Largest part of hub diameter: 19mm thrust washer portion
Smallest part of hub diameter: 12.7mm