The ZM1040 is a 30mm numerical indicator tube with red coated glass for better contrast. Due its large characters, it has become one of the rare tubes. The read paint on the tube was to reach better contrast. I dont know much about the orginally usage of these tubes, but i think they where probably used in measuremnt devices (e.g. Voltmer) just like the Z566M. This tube uses the B13B socket and is directly replaceable by ZM1042,Z566,Z5660 etc.