With the fins on it, that looks like maybe some sort of brake hub, maybe from a moped or similar powered vehicle, as opposed to a bicycle? If so, the missing wheel may not have matched it (they might have put the front wheel on one side, rear wheel on the other).
Edit: Here's some flywheels and freewheels that are similar:
http://www.itaous.com/listproducts.php?id=62
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Last edited by StephenH; 10-22-12 at 06:28 PM.