pressure relief valve
The large chunk of brass is the pressure regulator. On our '74 it has pretty much quit regulating, so I have had to put one on the supply hose outside the trailer. You can buy a new supply inlet with a built-in pressure regulator when you reinstall the plumbing (and toss or recycle the old regulator), if you so choose.
The smaller piece with the round top is a pressure relief valve. The copper line on the outlet side transitions to a vinyl or plastic tube which used to go through a small hole in the belly pan. I have replaced ours, and the technology is still the same. You should see a two digit number stamped on the aluminum disc on the top, which is the design relief pressure.