OK, that S/42 really had me going as the gun is clearly marked as DWM. Is there a date on the top of the front receiver ring?
The B in the unit mark is for Bavaria, which at that start of WWI was a separate Kingdom, though in a mililtary alliance with Prussia, the leading German state. "Germany" technically did not yet exist as a unified nation; it was still made up of several smaller states.
B. 4 R. M.G. 75 indicates the pistol number 75 of the machinegun company of the 4th Bavarian Regiment or, in the order on the gun, Bavarian 4 Regiment, Machinegun [company] # 75.