Hello, I'm knew to the forum, but I have lurked and enjoyed for some time. I recently came into possession of a Stevens 311 h with vent rib, 28" choked mod/full 20ga. Never heard of one like this and I am wondering how common are the vent ribs or if it's aftermarket ? It's a great field/small game gun that I will never part with. Just curious about it. Thanks in advance.