From the article:
The last major variant of the Sd Kfz 251 half-track was the Sd Kfz 251/22D (PaK40). Its existence came about as a direct result of Hitler's demand to mount as many anti-tank guns on vehicles as possible in an attempt to combat the ever growing numbers of tanks threatening Germany's boarders near the end of 1944.
The 7.5cm PaK40 anti-tank gun was mounted to the vehicle via a platform constructed to the rear of the driver's compartment. In order to accommodate the gun a few minor cosmetic changes had to be made to the half-track. These included cutting away a small section of the driver's roof in order to allow enough room for the recoil mechanism and the addition of a rudimentary travel lock weld to the front of the roof.
Since the gun was mounted in a relatively fixed position it could only be traversed around 20° to either the left or right. This meant that in order to engage targets that weren't directly in the gun's line of sight the entire vehicle had to be repositioned.
Two stowage bins provided storage for a majority of the ammunition with additional loose rounds kept in containers under the floor. The gun was fired by the gunner who sat on a small folding wooden seat located on the left-hand side of the gun with target acquisition made using the standard PaK40 sight. In addition to the PaK40, the Sd Kfz 251/22D retained its rear mounted machine-gun for self-defence purposes. Of the limited number of Sd Kfz 251/22 produced from December 1944, all were issued to the Panzer Division established in 1945.