The IN-4 is a clear glass end-view nixie tube manufactured by Gazotron. It is a perfect tube for end-view style clocks, because the body is smaller than the IN-1, and the tube is really common and available at several online stores. The "5" on the IN-1 is a inverted 2 like in many russian nixie tubes. There are two matching symbol tubes, the IN-7 and IN-7A