The British Army purchased several Polish-built 40mm Bofors guns in 1937, designating them Ordnance Quick Fire (OQF) 40mm Mk I. Suitably impressed with this weapon, a production license was acquired from Bofors and the gun was produced at Royal Ordnance factories in Britain. This British built gun was changed from metric to Imperial measurements, and several design improvements were made to facilitate mass production. This British built gun was designated the OQF 40mm Mk III, and became the standard light anti-aircraft gun of the British Army. It was designed to operate with the Kerrison Predictor, a mechanical analog computer which automatically laid the gun after simple calculations had been dialed in. a back-up sight ring and post, known as the "Pancake", was also provided. This was later used as the primary sight as the Kerrison proved to complicated to use in many situations and required constant fuel supplies for its electrical generator. In 1943 the Pancake sights were replaced by the Stiffkey sight which offered lead-correction, the sight was operated by an additional crew member. British guns were fitted with an armour shield to protect the crew from shell splinters and small arms fire. In British service the Bofors was usually towed by a 6x4 Morris tractor, later in the war the gun was mounted on a 4x4 Morris truck.