I'm thinking about late forties - round bout when we started identifying conductors. They started painting the neutral conductor. (Paint right over the plain jane black rubber and cambric or cloth) Prior to that you dont see too much in the way of identifying of anything. (Grounds neurtals or hots) I figure that from the practice of treating all conductors as hots - they didn't go as far as to actually identify them, but with the advent of actually grounding things figured you needed a method of doing so for ground and grounded...
Mark Heller "Well - I oughta....." -Jackie Gleason