The elongate rocker is forked at its first end and includes two fork tines. These are designed symmetrically relative to the longitudinal plane of symmetry of the rocker. A control rod with a longitudinal center axis is pivotally mounted at its bottom end to the second end of the rocker. A spiral pressing spring with a longitudinal center axis acts onto the rocker. The upper feeding roller is designed symmetrically to a plane extending perpendicularly relative to its longitudinal center axis. The longitudinal center axis of the control rod defines together with the longitudinal center axis of the pressing spring a geometrical plane. This plane coincides with the longitudinal plane of symmetry of the rocker, which in turn coincides with the geometrical plane which extends perpendicularly to the longitudinal center axis.