To counter the Russian T34 tank threat, the German Tiger I heavy tank was placed into service during 1942. Despite its excellent overall design, there were the inevitable minor mechanical problems found in any newly developed equipment. In addition, some improvements were suggested by combat experienced crew members. The most crucial of these was the replacement of the turret commander's cupola. The original cupola was rather tall, and its vision ports used bulletproof glass that provided only a limited view. From July 1943 onwards, a new cupola with seven periscopes and a side pivoting hatch, plus an anti-aircraft machine gun ring mount was provided. Starting in September 1943, a Zimmerit coating was applied to the vertical surfaces of the hull and turret, and a gun travel lock was added to the rear hull beginning from November.