Being a welder ive seen many adjustable regs with one guage, and ones with two guages you dont need 2 guages it just makes it easier to set if you have 2. It also enables you to see end of tank dump. If you need more pressure turn it up you dont need a second guage to know its working. Just like you dont need a bubble counter to set your flow or a rev counter in a car it just makes it easier.
That reg looks adjustable to me. Screw black knob clockwise and if it gets tighter and tighter its adjustable