Zimmerit coating was applied to all vehicles until September 1944. Production series First Production of the first series of 60 vehicles began in April 1943. Fifty-two of these were built using new Panzer IV Ausf. G chassis and the remaining 8 from rebuilt Ausf. E and F chassis. Survivors, about half, were rebuilt beginning in December 1943; they were mostly rebuilt to 2nd series standards. Second Production restarted in December 1943 with another 60 vehicles, using only new Ausf. H chassis, and continued until March 1944. The Sturmpanzer's baptism in combat at the Battle of Kursk proved that the driver's compartment was too lightly armoured and it was reinforced. The gunner's hatch was removed and a ventilator fan was fitted, much to the relief of the crew. Internally sprung, steel-rimmed road wheels replaced the front two rubber-rimmed road wheels in an effort to reduce the stress on the forward suspension that was only partially successful. Third Production of the third series ran from March to June 1944 with few changes from the second series. The Fahrersehklappe 80 was replaced by periscopes and the lighter StuH 43/1 was used. Fourth The superstructure was redesigned in early 1944 for the fourth series, which used the chassis and HL120 TRM112 engine of the Ausf. J, and was in production between June 1944 and March 1945. It featured a redesigned gun collar, as well as a general reduction in height of the superstructure. This redesign also introduced a ball mount in the front superstructure for a MG 34 machine gun with 600 rounds. The vehicle commander's position was modified to use the cupola of the Sturmgeschütz III Ausf. G, which could mount a machine gun for anti-aircraft defense. Combat history Sturmpanzer-Abteilung 216 The first unit to take the Sturmpanzer into battle was Sturmpanzer-Abteilung 216. It was formed at the end of April 1943 and transferred in early May to Amiens to train on its new assault guns. It was organized into 3 line companies, each with 14 vehicles, and a battalion headquarters with 3 vehicles. It arrived in Central Russia on 10 June 1943 to prepare for Unternehmen Zitadelle (Operation Citadel), the German attack on the Kursk salient. For this action, it was temporarily assigned as the third battalion of schweres Panzerjäger Regiment 656 ("Heavy Anti-tank Regiment 656") under the command of the 9th Army of Army Group Center. It remained in the Orel-Bryansk