I see not a sign of that "Circumferential Ring" in this bbl. An obstruction creates a localized high pressure area which will first cause a bulged ring "Around the bbl". If it is severe enough it will then burst at the weakest point, but signs of that ring will be present. It may be there, but I cannot discern it in this photo. All I see is it is just gaped open, but I see no "Stretched Area" around the bbl at any point.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra