The cap replaced the 'pillbox' style cap about the same time as the army changed theirs... so 1911 is a maybe, 1914 a definately. It was worn primarily by 'officers' (Inspectors upward), 'auxiliaries' and police officers permanently engaged in vehicle duties. Unless it's changed recently, foot patrol constables and sergeants had to wear the helmet (being allowed to take it off while being transported in a vehicle). The helmet had formed into the 'classic' style well before the turn of the century.