Large balance weight, including bracket and lever.
This weight is not based on any prototype, and is actually overscale, but is designed to give you the best chance of the lever automatically restoring the arm to the horizontal, particularly for garden railway use. If this still does not work, you will need to 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.)