The viewing window of a Montblanc 146 is frequently present but barely visible; with a strong light source (halogen or LED) behind the barrel, body and viewing window can be clearly distinguished by colour, since the window shimmers through brighter. On old 146s from the 1950s, the celluloid body can be discoloured dark red to black by aggressive inks, so that the ink window also appears obscured. Today, too, with use of certain inks a thin coating can form. For authenticity checking, comparison photos, proportion comparison with documented originals and the serial number help.