Ouestion on AC system

OK I got my AC system back in order and now have it on leak test. I have a question on a part and perhaps one of the AC experts on the site can give me a answer. I am trying to understand what the part that screws on to the fitting after the Receiver/Drier and has a small tube that runs in back of the brake MC. It then has a coil and screws to the top of the bulkhead. So what is this and what does it do. If I look into my 76-77 parts catalog in section 40 it is part #16. Does this attach to the evaporator ? What does it measure? Just trying to understand it's function. Thanks

 

OK thanks.