other free ends of the arms stop blocks 26 are welded or otherwise fixedly secured to the opposite side of the frame 16 as shown in fig1 and 2 each stop block is formed at its opposite ends with steps 28 providing clearances for extensions 30 formed on a channeled keeper 32 the construction of an individual stop block is illustrated in fig7 the extensions 30 are normally in the full line position shown in fig2 providing engagement between the extensions 30 and the adjacent wall surfaces of the stepped blocks 26 as a result the arms 20 are normally retained by the keeper in positions parallel to the plane of the back plate 10 and in these positions the arms will properly support a closure panel in its normal sealing relationship to opening 14 the arm 20 is of tubular formation being of rectangular or box like cross section and at the free end of each arm there is provided one of the above referred to keepers each channeled keeper 32 see fig8 is of c shaped cross section disposed normally in embracing relation to the distal end of the associated arm 20 keeper 32 thus is normally in the full line position of fig2 it is pivotally connected to the arm 20 through the provision of a pivot pin 34 so.