That looks like the Milwaukee MA-957 regulator (the regulator I use), but minus the bubble counter.
I found instructions for it online:
Operating Instructions for the Milwaukee MA957
CYLINDER CARE:
Secure cylinders so that they will not tip or fall.
Inspect the cylinder valves for damaged threads, dirt, oil, or grease. Remove dust and dirt with a clean cloth.
DO NOT ATTACH THE REGULATOR IF OIL OR GREASE IS PRESENT.
Oil and grease in the presence of oxygen is EXPLOSIVE!
Crack open the cylinder valve for an instant to blow out any foreign matter in the throat of the cylinder.
Use Teflon tape on the threads of the cylinder before attaching the regulator. Failure to do this can result in the loss of gas!
REGULATOR ATTACHMENT AND USE:
Attach the regulator to the cylinder valve placing the supplied plastic o-ring (not shown) between the regulator and the tank. Tighten with a wrench. Omission of this o-ring will cause loss of gas!
Unscrew the large brass ring (G) from the Bubble Counter (H) and fill the bubble counter 1/2 full of fresh water. Be sure the needle valve (B) is closed before filling with water. Once filled, replace the brass o-ring (G).
Before opening the cylinder valve turn the regulator adjustment knob (E) counter clockwise until tension is no longer felt on the knob. Do not fully remove the regulator flow adjustment knob. IF THE CYLINDER VALVE IS OPENED WITH THE REGULATOR KNOB CLOSED, DAMAGE TO THE REGULATOR CAN RESULT!
VERY SLOWLY and CAREFULLY openthe cylinder valve COMPLETELY to seal the cylinder valve packing. The amount of gas in the cylinder can now be read on the High Pressure Gauge (D).
Plug the solenoid (C) into a suitable, grounded, AC Wall outlet, Hanna pH Mini Controller, or Milwaukee SMS122 pH Controller. The solenoid uses slightly more then 6 watts and it is normal for it to feel warm during operation.
Open the needle valve (B) by turning counter-clockwise.
Turn the adjustment knob (E) clockwise until you get a reading on the Low Pressure Gauge (F) of approximately 10lbs on the outside set of numbers (use the numbers on the top row). You should now see bubbles in the counter.
Unplug the Solenoid (C) from the wall or controller and unscrew the compression fitting and pass the air hose through the fitting and attach the tubing to the
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(A). Be sure your tubing fits tightly on the
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and that it is pressed all the way down. Slide the compression fitting onto the
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and tighten.
Plug the Solenoid (C) back into the wall outlet or pH Controller unit and adjust the bubble count on your reactor using the needle valve (B). Fine tuning the count can be accomplished with either the needle valve (B) and by adjusting the regulator output pressure with adjustment knob (E) as discussed in the pevious step. The more you increase the pressure as indicated on the low pressure gauge (F) the more difficult it will be to fine tune your bubble count using needle valve (B). Therefore it is more desirable to keep the pressure indicated on the low pressure gauge (F) low and adjust your bubble count using the Needle Valve (B).
And here is a labeled picture that corresponds to the letters in parentheses in the instructions: