Very nice picture. What is it that he keeps at his neck? Seems like some kind of instrument. Maybe a portable range-finder?
I suppose your grandfather was an officer, wasn't he? (spike and decoration on the helmet).
I'm not sure what that is, and I am not sure what level he attained during service, only that he was killed in action, and left a widow and 2 small boys, one was my father, who would be 101 now if he was still with us.