The Rega Tungsten counterweight has the same mass as the standard counterweights for the RB300, RB301, RB303, and the RB330, but it will actually yield improved sound performance due the higher density of the tungsten. The higher density makes for a smaller diameter weight, that places the mass closer the centerline of the arm tube improving tracking and balance. There is an additional side benefit of the smaller diameter, if you choose a non-Rega cartridge that requires spacers (spacers will lift the arm and the stock counterweight may contact the inside of the dust cover), the smaller diameter of the tungsten weight will provide the extra clearance needed inside the dust cover.