To easily move a jaw position and to make a jaw supporting and holding part able to hold automatically and surely in the direction of a toothed rod axis by making at least one of the jaws able to handle an object with a different thickness and by connecting the jaw adjustable lengthwise to its corresponding toothed rod.

System height adjustment to a new packaging is done either by removing from a rod 34 a wedge 56 energized toward a rod tooth by a spring 58 and by rotating a small gear 42, or by moving a bottom jaw 20 along the rod 34, bringing one of the jaws 18, 20 to a height i, h/2 above a carrying height, then bringing the other jaw to the same height and by fitting the wedge 56 to the rod 34 once more. Then the driving system of the small gear 42 can be operated to move the jaws 18, 20 in the proper open position symmetrically facing the passage of a product. A method for using a holding member 66 is also desirable.