Phototypesetting disc. As the Fototronic disc spun at a high speed, a light would flash at the correct moment to expose the desired character onto photopaper. The two rows of thin radial lines identify the characters. There is space for seven lines between every pair of longer lines—I'm guessing this corresponds to the encoding on the punched tape used as input. Not sure about the radial columns of squares. These are not unique across the font. Something to do with spacing?