The gun in question is the 10.5cm L45 (that's the calibre). This became the standard deck gun for u-boats in 1918. U 91 was commissioned with this armament, other u-boats with the older 8.8cm L30 guns were mostly retrofitted with the 10.5 in 1918. The gun was a an adaption of the 10.5cm anti-air gun for surface ships. The mount was streamlined and allowed for elevations from -10 to +50 degrees, thus making it theoretically possible (but impractical) to engage airborne targets. Gun elevation and traverse were controlled by a single gunner, who also fired the gun by pressing a lever with his knee. The projectile weight was 18kg and the muzzle velocity 650 m/sec.