Common round 20lb balance weight, including bracket and lever. Spare weights are SC041/1.
Due to the friction inherent in 7mm scale bearings, you may find that the lever will not restore the arm to the horizontal. Try using SC044/1 weights instead, or configure the operating mechanism to move the arm in both directions. (Note that on the prototype, the function of the balance lever is to keep the wire from the signal box taut. If anything breaks, the difference in weight between the arm and the spectacle casting restores the arm to danger/caution.)