The flute or whatever it's called is quite common, although it generally seems to be more common on American made guns or guns made overseas for the American market. Every one of my Superposed guns has the feature, as does my Win 101. But not my FN A1 grade Super, made for the world market. Looking through photos in my meager gun library and on the web, I see it on about 2/3 - 3/4 of all shotguns. Even rifles. Very unscientific survey, I'll admit. As common as it is, I'm surprised very few know what it's called. I never wondered about it before now. I bet Doug Mann knows.