Centurian , the gun is the 7,62 L/30 "Lenders" M1914. Originally intended for ship mounted anti-balloon use, stocks were diverted in 1915 for installation on US White or Peerless lorry chassis. The vehicles appeared armoured and partially amoured. ( I have a pic of several in a captured vehicle park.)
The gun went on to serve in it's original mode as a ship's mounting, and apparently proved to be quite succesful as it's image appears on Russian captial ships mounted atop multi-gun turret main armaments.