According to a product description in Track of the Wolf, "Original longrifles were nearly always fitted with "fine" (low) sights, in the mistaken belief that the low sights reduced sighting error." I'm not sure why that would have been the belief, but it may be that in the old days nobody saw the need for a '6 oclock hold' and a 6 oclock hold is where deep notched rear sights and square front posts shine. And, except for combat situations fighting Injuns or redcoats or whatever, one shot was probably all you were going to take, so heat mirage wasn't a factor.