Not exactly to the point of your question, but John Garand called it a "head positioner." You can't focus on three things at once, so the rear sight was intended just to make you put your head in the right place to focus correctly on the front sight and the target. Pretty cool. FWIW, almost no GIs I ever saw shot with both eyes open except for snap shots at close range. In qualification with the M1icon I got one close silhouette from the hip a split second before it went down