This is the barrel style starter drive gear which is available for the Model A & B. It replaces the Bendix style which are prone to breaking springs and to losing bolts in the flywheel housing.
This style uses an inertial throw-out on a helix, and the shock is absorbed by a rubber doughnut in compression rather than a coil spring in tension. The engine speed disengages the gear from the flywheel after the engine starts.