I can't see everything so I apologise if my assumptions are wrong.
I've assumed the vertical pipe to the left of the clock goes down to the bottom horizontal pipe, if it doesn't then you can ignore my post.
Looking at it now I guess that vertical goes to a drain point
That's fine, yep that is a drain at the bottom of the vertical pipe.
So there's another T joint hidden behind the tailstock and D is connected to the drain via that cheeky pipe circled above?