In a receipt printer (10), suitable for printing on a paper ribbon of different widths, wound on a roll (14) arranged in a seat (15, 22, 23, 24) in the printer, a wall is provided for the lateral positioning (18, 30) of the roll of paper (14) in its seat, to keep the paper ribbons, or the respective rolls (14a, 14b) of at least two different widths in the proper printing position, independently of the width of the ribbon; the positioning wall (18) is arranged inside the seat (15) of the roll of paper, with a first surface (44) facing a side (26′) of the same roll (14a). When the roll (14a) is replaced with another roll (14b) of different width, the positioning wall (18) is repositioned in the seat (15, 22, 23, 24), rotated through 180°, to laterally retain the new roll (14b) with the second face (45), opposite the first face (44); the positioning wall (18) is suitable for being attached to the walls (22, 23, 24) of the seat (15) through flat tabs (38, 39, 40), protruding beyond the edge (32) of the positioning wall (18); the positioning tabs (38, 39, 40) are all arranged staggered on one side (44), with respect to the median plane of the wall (18) and flush with one (44) of the two faces of the positioning wall (18) and are also suitable for engaging corresponding pairs of slots (41, 42, 43) formed in the walls of the seat (15).