Those "dark lines" are caused by the shadows from the damper wires used in any kind of aperture grille (AG) monitor. 15, 17, and 19 inch models have two damper wires, and above 19 inches, there are usually three damper wires. Most people do not even notice them after a while, your eyes get used to them. They are most apparent with lighter backgrounds.
Also, Trinitron = Sony, DiamondTron = Mitsubishi, and SonicTron = Matsushita. These are the three major CRT producers for AG monitors.