This is a replacement O.S. 6.0mm Aluminum Carburetor Reducer, with included o-rings. This restrictor comes included with the B2101 engine.
The carburetor restrictor (or venturi) is used to adjust the flow of air into the engine. By changing the size of the venturi, you can fine tune the power delivery and run time of the engine. A larger venturi will increase top end power and is better suited to larger tracks, but will reduce run time. A smaller venturi will improve bottom end throttle response making them a good choice for small tracks, and will help to improve run time compared to a larger venturi.