"By the way, could you have a look at the photo in #6 and tell me what the green, black, and white fittings are used for in the upper right quadrant of the photo?"Pop Quiz...
Just looking at it from 20,000 feet away, it looks to me like:
Horizontal Top - Simplex check valve.
Horizontal Bottom - (Attached to the galv. fitting) Possible pressure relief valve.
Verticle Right - Possible air relief / vacuum breaker
White Tank - Hydropnuematic tank for some purpose, possible surge protection or pulse dampening. The main tank may be fed by a reciprocating (piston) type pump or pressure boosting centrifugal pump.
What's the real story?
This is the view in the water heater closet at my apartment. I wasn't sure of the real story, which is why I asked. The larger diameter pipe in the back corner of the closet is the riser that goes to the two apartments above us, and you can make out the water meter in the lower left behind the hose bibb.
The green thing is apparently a dial, it has numbers around its perimiter - perhaps a mixing valve? The pipes make a loop between the cold and hot pipes of the water heater.