The ZM1042 is a clear glass indicator tube with characters of 31mm. Due its large characters, it has become one of the rare tubes This tube was produced by multiple factories like Valvo,Tesla and Philipps. 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.