The outlined frame of packing introduces a sporgente ribbing (12) that it constitutes a point of held (13). Since but this must be leaned on the rectilinear edge of the chassis (1) and therefore it finds an estate too much insufficient, to foot (18) of the outlined frame of packing (4) is arranged an organ of anchorage (19). It introduces labbri (21) decided oblique sporgenti ones towards the high that forms a dentellatura with which the organ of anchorage (19) can be leaned around inferior chine (20) of the window slab (2) and in this way existing between the chassis (1) and the slab can withhold the outlined frame of adjacent packing (4) in the commessura (3) (2).