the thickness of said second arm 4 a second plate 5 pivoted under the front end 4 u2033 of said second arm 4 by means of a fourth pin with vertical axis p 4 and also provided with suitably shaped profile to be exactly engaged into the traditional longitudinal groove s 1 that is defined in correspondence of the upper side of the cross piece of said shutter im by a parallel pair of stiffening ribs c 1 it being provided that said second plate 5 is provided in correspondence of the back end with a lowered longitudinal step 5 a facing the side that corresponds to the one where on said second arm 4 said lateral projection 4 c is situated moreover said first plate 1 is firmly fixed inside the groove defined by said two ribs c of the cross piece tt of the frame t by means of a locking pin g mounted in tilted position as expressly shown in fig8 the knee lever mechanism l is adapted to cooperate with a latch 6 u2014 mounted in vertical position on the outside of the upright mim 2 of the shutter im in the proximity of the hinges u2014 which is an integral part of the new shutter holder of the invention the back end 3 u2032 of said first arm 3 is adapted to rest above said wing 2 the distance between said second pin p 2 and slot 2 a in said wing 2 is exactly equal to the distance between said first hole 3 c and said second.