Gas can be manually exhausted through the exhaust port, via a hand knob, or with a chin button inside the helmet. The chin button is just a spring-loaded valve that you operate by moving your head inside the helmet, and pressing on the button with your chin. I think this is probably because it’s easier than taking your hands off your work, or trying to operate the external valve with heavy gloves. Although, you can certainly turn the knob externally.
Exhaust gas travels from the inside of the helmet, through the valve and along the exhaust port, a cylindrical fixture mounted to the side of the helmet that directs the flow around the helmet and exhausts it out many small holes drilled in the end of the port.
(Photo found at Divingheritage.com)
I’ll start with the exhaust valve handle. It’s basically a star-shaped square, with a ball at the end of one of the points. I’m making it out of polymer clay. It takes several attempts to get a close enough replica. There is the handle, a boss to keep the handle separated from the exhaust manifold, the chin button, a spring to allow the chin button to spring back to it’s original position, and the shaft to connect everything together.
Several attempts to get the shape correct. 
The exhaust valve and chin buttons. 
Handle and boss test-fitted. 
Gorilla glue and clamp. 
Test-fitted exhaust valve and chin button.