This is the Great Planes ElectriFly Collet Cone Adapter 5mm-5/16x24.
Aluminum construction, silver in color. Mounts on electric motors with a 5mm shaft and adapts them to a 5/16 x 24 diameter prop shaft. Cone shaped spinner nut adds good looks.
Length : 1.82 in. (46.3mm) as assembled
Length : 1.94 in. (49.5mm) shaft only
Length : 0.94 in. (30mm) spinner nut only
Shaft Diameter : 5/16x24
Spinner Nut Diameter : 0.71 in. (18mm) at the widest point