The disclosure is directed to an apparatus for marking on planar material, selected indicia associated with the shade of the material passing therethrough. The apparatus comprises a housing and a printer assembly having two parallel circular end plates positioned opposite one another movably disposed on two parallel opposite walls of the housing. The plates are each fixed at their centers of rotation to either end of a central slightly flexible torsion shaft. Three parallel rollers are rotatably mounted between the plates. The rollers are preferably equally spaced from one another and to the central shaft to form, in cross section, an equilateral triangle. Spaced along the rollers and parallel to the end plates are idler plates for engaging the moving material to rotatably drive the printer assembly. The idler plates contain central apertures larger in diameter than the torsion shaft so that the assembly may flex during operation. A plurality of pairs of raised indicia carrying belts are mounted about the rollers on sprockets disposed on the rollers, so that one of each of the pairs of belts frictionally engages one such roller and the other of each of the pairs of belts frictionally engages another of the rollers. Each of these two belt engaging rollers can be independently turned to position selected raised indicia on each of the belts at the apexes of the equilateral triangles formed by the rollers. An inker inks the selected raised indicia. As material moves through the housing, it rotates the idler plates and hence the assembly so that the printer assembly marks the material by contacting the material passing thereunder with the inked selected raised indicia of the belts positioned at the apexes of the triangles.