I'm interested in your opinions.
My opinion is...a primitive WW1 blackout mask. It has an adjustable weight in the flap. As the motorcycle moves forward the flap opens and when the motorcycle stops the flap shuts.
email (option): allanpainter@netspace.net.au
Looks like a stove pipe draft to me.
Its some sort of vent. The motorcycle moving forward wouldn't keep the flap open. once the weight reached the speed of the motorcycle it would close.
Looks alot like some sort of vent cap. It attaches to the side of an exhaust pipe or stove pipe. The pressure build up holds it open and keeps air from coming in. once the pressure falls the lid shuts and prevents anything getting in the pipe.
Very much identical to the modern vent caps I use in construction today. I call them vent flappers.
Edit: not sure if it pushes open or pulls in. But if it is the reverse of my description its most likely some sort of intake cap. Air pressure drops (whatever its hooked to begins to draw air) it opens and when it stops the flap shuts. Keep birds out 
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