No offense meant for the following:
I love these threads. They make me smile when someone asks a question that has them confused and it is a situation that others have seen and dealt with many years ago.
Yeah that valve is similar to what you see on your wheels. They are Schrader valves that control the air in the system. Just like with a car tire, you press the center pin and the air comes out. You press an air hose to it, the pressurized air goes in.
"Air shocks" were popular many years ago as a means to raise the rear of a vehicle for a few different reasons. One was for style. Some people loved the look of a raised up rear end of a vehicle. Some people wanted to put wider tires on the car and when they wouldn't fit, the car had to be raised for clearance. Some people used them to allow them to tow a boat or to carry more people or cargo when the stock leaf springs sagged too much under load. They were popular but lost their appeal sometime in the early 80s.
I've used air shocks before. One drawback is that the mounts in our cars were not designed to take suspension loads, they were designed support the shocks that only dampened the movement of the suspension. This meant that the mounts are not thick enough to tolerate the demands that some people put on them. Under duress, they will sometimes deform or come loose from the frame.
The proper fix for sagging springs is to replace the springs. You can have leaf springs made to handle additional load capability.
Cheers and good luck.