So- just clarifying -- you're saying those threads are an adaptor? And can be removed?
That is my interpretation of what your photo shows, and so far no one has disagreed. I believe the pipe behind the fat ring is 1/2" nominal copper pipe, with a soldered joint showing just outside the wall.
You could confirm that hypothesis in a few different ways:
1) Get some medium abrasive (120+ grit sandpaper, scotchbrite, etc) and clean up the surface of the pipe to see if it's shiny copper.
2) Measure the diameter of the pipe--it should be 5/8" if is it 1/2" nominal copper.
3) Look closely down that hole in the fat threaded ring with a flashlight--does it look there's the head of a set screw there?
If (3) shows a set screw, and (1) shows copper pipe plus some shiny silver near the joint, which is solder, and there's no solder near the fat threaded ring, then loosing the set screw should allow the ring to come off. It might require a little force, if there are hard water deposits that have cemented the ring on a little.
Cheers, Wayne