bottom 14 of the groove 11 as particularly well seen in fig4 the projection forming contour 23 deviates smoothly from the line 24 to thus provide for a gradual transistion into the projection 26 in the closed state of the latch 7 an engagement face 25 of the latch 7 lies in contact with the groove bottom 14 the stepped portion 22 begins preferably immediately at the end 12 of the latch spoon the latch spoon end 12 projects into the groove 11 when the latch is in its closed state in this manner the stepped portion 22 covers and thus shields the hook tip 4 so that the thread is deflected and cannot contact the hook tip 4 in the closed state of the latch 7 the projection 26 which preferably starts at the groove 11 and allows the inner contour 23 of the latch 7 to merge smoothly into the inner edge 17 of the hook 3 thus the contour 23 may be regarded as having two adjoining length portions a first length portion which coincides with the imaginary line 24 and a second length portion constituted by the projection 26 and situated between the free end latch spoon end 12 of the latch and the first length portion such a smoothly merging configuration may be particularly well seen in fig5.