The chevrons are for dress blues circa WWII or earlier, without the crossed rifles or diamond in the center (for First Sergeants). This was before they made the "super sergeant" ranks that included Master Sergeants and Master Gunnery Sergeants. A Corporal in those days was an E3, though he DEFINITELY was an NCO, and the Sergeants were E4's. From what I understand, Sergeant Major was more of a title than a pay grade for quite a while as well. Going back even further, a First Sergeant was a title rather than a pay grade as well.