This gasket has barbs that match similar notches in the extrusion. This gasket is intstalled on the inside of the window after the exterior has been installed. A little soapy water and this wedge shaped gasket with push in and give tension against the exterior gasket and also means your acrylic is cushioned between two layers of rubber. Although I have seen some older windows without this gasket seemlingly from the factory. Perhaps they would run out and move on without them. Hard to say but I use them on every window I build.
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