device on the frame of an opening thanks to brackets 5 and releases shell 8 in order to access the spring box 3 it then suffices to connect the lower end of the strap 2 with the box 3 and to remove a clip 9 immobilizing this box before sliding the shell 8 in the direction of the arrow f 1 in order to close the box formed by shells 4 and 8 the end of the movement of the shell 8 resulting in a locking of the box in closed configuration by cooperation of shapes of elements 43 a 43 b 83 a and 83 b taking into account the usual orientation of the brackets 5 the edging 43 of the shell 4 is intended to face the partition against which the winding device 1 is mounted this has the consequence that the tongue 43 b and the orifices 43 a are not immediately accessible in particular for a child which avoids the risks of an unauthorized opening when a professional wishes to open the box formed by shells 4 and 8 it suffices to pivot the shell 4 about the articulation formed by the rivets fixing the brackets 5 in order to access the tongue 43 b on which he may exert an effort of retraction of the beak 43 f as represented by arrow f 2 in fig7 the invention has been shown with the shell 4 bearing the elastic lock 43 b however such a lock might be provided on.