WW2 RAF Mk.II Helmet - OP 4

An interesting MK.II helmet. The outside of the shell is grey in colour and has OP4 stencilled to the front in white whilst the inside retains its factory finish. I am not sure exactly what OP4 stands for, possibly Observation Post 4 or Operations 4. I am 'presuming' this belonged to either a member of the RAF or the Observer Corps? The is shell stamped ROCO 1939 with 1938 stamped lugs. The liner is stamped J.C.S & W Ltd 7 1939 or could be 1938 and is in fantastic condition. Any further info on identifying the exact usage of this helmet would be greatly welcome.