Common round 20lb balance weight, including bracket and lever. Spare weights are SC0041/1.
Due to the friction inherent in 4mm scale bearings, it would be most unusual (and a tribute to your workmanship) if the lever restores the arm to the horizontal. Instead, the operating mechanism must be configured 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.)