The inner chamber wall has a small hole that the screws seat in. From what I can see they are beveled like the screws to be able to seal when fully screwed down.I just realized I have no idea what is going behind those 3 screws....just curious.
Yeah I tune mine on the hose before install. The screw settings are always different for each pipe.Its just a water jacketed pipe with the adjustable water screws. They flow quite a bit when you open up 1/8 turn. Can see that if you hook a garden hose to headpipe. This older model, the bottom screw never actually closes, always flowing some water, when it's shut it flows the equivalent of 1/8 turn open on top screw. So its not a bad idea to check water flow occasionally and see how it reacts to screw settings.
FPs recommended settings are just the starting point. That's why you have to check temps when you turn them in.Interesting!