What you have is called a single stage, dual gauge regulator. From the pic, like Ron said, the right gauge is the CO2 cylinder pressure, and the left gauge is the baseline pressure at which CO2 is being delivered to the bubble counter or wherever. There should be a minimum pressure that this never goes below, like maybe 5-6 PSI. What you will see is that the left side gauge can rise when the regulator solenoid is off, then after the solenoid is energized it goes down to the baseline PSI reading (5-6).
The baseline delivery PSI is set by rotating that rusted slotted center screw at the bottom of the pic in the center of the "UNIWELD" logo clockwise to increase the baseline PSI or counterclockwise to decrease it. But the true baseline delivery PSI will be seen when the solenoid is energized and has been on long enough to lower the pressure to the baseline delivery pressure rate.