Goes along with the internal flange and inner locking band (with centering extension). The outer locking band is on the surface for more aid, which makes it a more robust system. Reduction in cross-segment: The interior dimensions (width and height for rectangular ducts, diameter for round ducts) on the smoke https://fire-resisting-ductwork-i67789.pointblog.net/detailed-notes-on-fire-resisting-ductwork-insulation-83304471