These are guesses.
How about it was some kind of control system damper. It whizzed round when some flight control was moved providing:-
a) Aerodynamic resistance - hence the vanes
b) Inertia - hence the extra bits riveted on.
Hmmm - not very likely really.
Or:
It is the hub of a larger paddle used as a centrifugal flow fan or simply an aerodynamic brake. I think I recall seeing such a brake on a parachute jump simulator on TV. e.g. Fifty foot tower, strap in, jump off, harness and string turns fan which provides controlled descent.
Basic Parachute Course - Wikipedia, the free encyclopedia
"The next stage involves the fan trainer. Wearing a harness connected to a cable wound round a drum fitted with fan blades, trainees jump from a platform located near the roof of the hangar; as they fall, their rate of descent is controlled to simulate that with a parachute."
This one was last sold in 2005.
Some keywords.
RCD Fan Descenders
Redpoint Descender
RCD POWERFAN
Here is the Mark II which looks more confidence inspiring to me:-)
Ropes Course Developments - RCD Linear Ropes Course