Twice the minimal wit because posted twice? I'd bet Chuck is on to something; one per gun ain't enuf. I've always seen them associated with target guns and all the talk of "figure 8" picture and "superposed" picture would lead one to believe that they are intended as a telltale of where the gun might be expected to shoot in terms of elevation given what one sees after an eyes-closed mount check or check mount or whatever. Being more or less adept at actually thinking in two dimensions at once, I've also used them to check lateral alignment of the eye over the rib after what for me passes as a reasonably competent but stiff "Susan B. Anthonys still on the rib" practise mount. Then there are all these swamped rib or no rib guns where you got to pick out the centerline as a depression between the barrel balls or as the rib extension or a centralized swale or notch in one sort of abbreviated rib or another. All this changes when something flies: swing gun, sense daylight between object of your desire and that other combination of things you're not looking at or your pointing finger or thumb; pull trigger. Most of the time the butt hits my shoulder sometime in there; I'm never quite sure when. Wherever I'm looking, I always hit what the gun's looking at even if it's only thin air.
jack
