This is the Great Planes ElectriFly Collet Cone Adapter 4mm-1/4x28.
Aluminum construction, silver in color Mounts on electric motors with a 4mm shaft and adapts them to a 1/4x28 diameter prop shaft. Cone shaped spinner nut adds good looks.
Length : 1.61 in. (41mm) as assembled
Length : 1.31 in. (33.3mm) shaft only
Length : .94 in. (24mm) spinner nut only
Shaft Diameter : 1/4x28
Spinner Nut Diameter : .59 in. (15mm) at the widest point