In order to ensure that the abutting edges are permanently and securely held together, {TAIL ~ the abutting edges} of at least the outer material layer, preferably of all material layers, have contours which are toothed and complementary relative to each other and inter engage in {HEAD ~ circumferential direction} of the hollow cylinder 
In order to ensure that the abutting edges are permanently and securely held together , { TAIL ~ the abutting edges } of at least the outer material layer , preferably of all material layers , have contours which are toothed and complementary relative to each other and inter engage in { HEAD ~ circumferential direction } of the hollow cylinder
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