9 has been rotated by the key to the position shown in fig3 at which position the key can be extracted from the keyhole then the lock is in its closure position since the annular groove 205 of the bolt 5 is engaged by the cylindrical surface of the plug 9 which projects at the interior of dead end bore 4 so that the said bolt 5 is locked against axial displacement and it engages the anchoring ring a see also fig4 which is secured to the threshold of the door to be closed by the roller shutter in order to open the lock it will be necessary to introduce the key and rotate the plug 9 so as to bring its flattened portion 109 in alignment with the inner wall of bore 4 in this manner it will be possible to extract the bolt 5 thus disengaging same from the anchoring ring a as it appears from fig3 and 4 a bore 15 is obtained in the thickened portion 601 for housing a detent pin 16 which is urged towards the exterior by the spring 17 the said pin cooperates in the closed condition of the lock with an annular groove 305 provided in correspondence of the free end of the bolt 5 so as to hold steady the bolt 5 in its inserted or closure position.