This counterweight counteracted the muzzle preponderance of the gun and cradle due to the trunnions being displaced to the rear. The counterweight is an iron casting attached to the rear of the cradle by a supporting beam and two supporting arms. The iron casting contains several compartments containing approximately 920 lb (420 kg) of lead.

The decision to manufacture the 3.7-inch anti-aircraft gun in Australia was made as a result of Imperial Defence Conference in May/June 1937 as a contribution to Empire defence. In October 1937 the Munitions Supply Board sent two of its officers, including Mr V. Parker, the Assistant Manager of the Ordnance Factory, to England to study the manufacture of the 3.7-inch gun, and at the same time the purchase of machine tools and plant was authorised. The first production order for 130 guns was received from the Army on 22 March 1939.  Ten months after receiving the drawings the Ordnance Factory turned out its first gun in May 1940. This was the beginning of the factory’s large scale contribution to the stock of weapons used in the war of 1939-1945. In August 1940 four guns were shipped to Darwin. Others were sent to Britain and set up in Hyde Park, London where they played their part in defending the city in the great raids of 1940. Other guns were sent to Singapore, only to be captured by the Japanese.