The device comprises side guide plates (12) which have a recess (15) extending to the lower edge (14). A part side-plate (17,18) aligned with the side surfaces is suspended in one position in the recess and is movable to another position in which it projects over the lower edge of the remaining part (13) of the side guide plate. The recess is in the form of the segment of a circle and the part side-plate is made as a disc mounted to swivel eccentrically in the recess.
