Concerning the P.R. 30 marking on a M1915 helmet, it may actually be Pioneer Regiment 30. Upon mobilization at the beginning of the war the 8 fortress battalions which existed in peacetime were expanded into 10 poineer regiments numbered 18, 19, 20, 23, 24, 25, 29, 30, 31 and Bavarian. In 1917, the original pioneer regiments were converted back to battalion size units.
The source for this information is the British Handbook of the German Army in War dated April 1918.
Inreresting helmet.
Reservist1