vent. The B model, which began production in 1940, eliminated the fighting compartment's side vision slits. The C model, which started production in mid-1942, featured a simplified hexagonal-shaped forward armored plate for the engine. Models A through C had rear doors that bulged out. The C model had a large production run, but was quite complex to build, involving many angled plates. From early 1943, the D model was developed with the purpose of halving the number of angled body plates, simplifying the design and thus speeding up the production. D models can be easily recognized by their single piece sloping rear, with flat doors. The standard personnel carrier version was equipped with a 7.92 mm MG 34 or MG 42 machine gun mounted at the front of the open compartment, above and behind the driver. A second machine gun could be mounted at the rear on an anti-aircraft mount. Variants were produced for specialized purposes, including with anti-aircraft guns, light howitzers, anti-tank guns and mortars or even large unguided artillery rockets, as well as a version with an infrared search light used to spot potential targets for associated Panther tanks equipped with infrared detectors. A strong design feature of the Sd.Kfz. 251 was the large track area, with the characteristic "slack track" design with no return rollers for the upper run of track. The Sd.Kfz. 251 also had the Schachtellaufwerk system of overlapping and interleaved main road wheels common to virtually all German halftracks of the period. This lowered the ground pressure and provided better traction, at the cost of much greater complexity in maintenance. The Sd.Kfz. 251 also had tank steering, whereby the normal steering wheel moved the front wheels, but after more turning of the steering wheel, the tracks are braked to cause turning, like on a tank. However, the interleaved and overlapping main road wheels shared a major problem with the Tiger I and Panther tanks that also used such roadwheel configurations - in muddy or winter weather conditions, such as those during a mud season (rasputitsa) or the winter conditions, accumulated mud and snow could freeze solid between the road wheels, immobilizing the vehicle. Use The early production models of this vehicle were issued to the 1st Panzer Division in 1939 as the 'Ausf.A' or 'A-version'. There were 232 estimated Ausf.A units produced, which took place in Hannover's Hanomag, Büssing-NAG of Berlin Oberschoeneweide, Weserhuette of