A ceramic main body 1 is composed of a 3O4\u2014 or 3O4-based ceramic material. The blanks, whose inner edges overlap, are guided in frictional engagement to their folding position above the folding shafts, whereupon the longitudinal edges of the overlapping blanks facing each other are, lifted together, whereupon the blanks are released by releasing the frictional engagement. Then, an identifier of a document representing the group is reproduced graphically andor in a sound manner.