This is one of the early prototypes of the Dekatron Simulator. As you can see from the video, the Dek-Sim on the right, triggers the Dek-Sim in the middle once it has reached the zero postion, and the one in the middle triggers the one on the left. This version of the design required each of the Dekatron Simulators to be powered and a wire connecting the output of one into the input of the another. In the new design, the connectors on the sides of the Dekatron Simulator are designed to pass ground, vcc, and the ouput to the Dek-Sim on the left. This will make it easier to daisy chain the modules to create displays for devices such as frequency counters with minimal wiring.