For future reference, this is a High Pressure air regulator (in this case on a 3000psi cylinder, but a 4500psi is also about the same but starting st higher pressure)
not in sight is a fill nipple (see picture 2)
the bottom nut is the high pressure burst disk (effectively a fuse in case too high a pressure of air is filled)
Next in the centre the pressure gauge with internal cylinder pressure
Then inside is the regulator wotkings stepping down
On the right is the low pressure burst disk in case the regulator fails
Final picture is the valve on a co2 cylinder
That does nothing other than release the co2, and can either have (1) just a pin valve to release co2 when connected, (2) just have an on/off screw to release co2, or (3) have both pin valve and on/off screw
The co2 is naturally released at aporix 850psi in it gaseous state, which is why the original air regulators were set at 800-850 psi