This is a Great Planes ElectriFly 5.0mm - 5/16-24 prop drive (adapter) with set screw and prop nut.
Requires: Installation onto the motor or gearbox shaft
End to end length: 1.6 (42mm)
Complete shaft length: 1.1 (28.6mm) with 0.84 (21.4mm) threaded portion
Prop shaft diameter: 5/16 (7.9mm)
ID for motor shaft diameter: 0.19 (5mm)
Largest part of hub diameter: 1.1 (30mm) thrust washer portion
Smallest part of hub diameter: 0.51 (13mm)