The fahrbare Panzerlafette, ("movable armour carriage", later shortened to Fahrpanzer, "mobile armour") was a 5.3cm Gruson quick-fire gun mounted in a armoured turret, served by two gunners. (Quick-fire gun denotes simply that the gun used a quick breach and ammunition where the shell and the cartridge were one item, speeding up the loading process. The gun itself had an elevation of -5° to +10°, and could fire a 1.75kg shell with a muzzle velocity of 495 m/sec. It could fire a maximum of some 30 shells per minute.) The Fahrpanzer was in use from 1890, originally a piece to be used in the German border fortresses: the Fahrpanzer were used in special concrete trenches with 60cm narrow gauge railway, making possible a quick movement of these mobile turrets: they were to be in shelter when not in direct combat, thus protecting them from the heavy artillery fire of the attackers, being wheeled forward to their combat position only when the actual assault began. It was a novel idea, and the Fahrpanzer was sold to several other countries before WW1, for instance Greece. It could also be moved along the roads on a special horsecar, seen below:
The horsecar was used on the export versions. The German Army used a slightly simpler type of transportation method, as can be seen in the third photo above. After the start of WW1, when the German Army, like all other Armies, found itself lacking artillery, it, again like most other armies, took to removing guns from inactive Fortresses and using them in the front lines. So many Fahrpanzer’s came to be used in Trench positions as well.
The Fahrpanzer below can be seen in the excellent Army Museum in Brussels