In early design, the gun mount was located on the floor of the personnel compartment. The commander's seat on the right as well as the frontal armourplate were removed. This newly created space was occupied by the 7.5 cm cannon, extra armor shield were also added on both sides of the gun for personnel protection. In this layout, the aimer and the loader were located side by side on the left hand side, resulting in some difficulty in operation.

A later version, using the Sd. Kfz. 251D chassis, was introduced in 1944. The gun was moved from the left to the center, and the loader seat was switched to the right of the gun. The gun, as well as its armor, were moved up to the roof of the personnel chamber. Although this modification had increased the height of the vehicle, it no longer required major modification on the front of the chassis. This new version was designated K51 (Sf), which was able to fire grenade and anti-armor shells. An axis MG42 machine gun was also added on the right of the main gun. Following the removal of the gun mount, the floor space had become more spacious. The Sd. Kfz. 251/9 was mainly served in panzergrenadier units, providing fire support for infantry units in all fronts.