The Mouat Water Seal Return Fitting
The early Mouat steam systems didn't have thermostatic radiator steam traps on the outlets of the radiators. What they had were water seals. Since the later version of Mouat systems had thermostatic traps, its easy to confuse the early systems outlet water seal return fitting as a trap also. It is by no means a trap.
I had cut a couple water seal return fittings in half so you can see the insides and made a short video explaining these devices and put it on you tube.
Just remember that if a radiator has water seals on its outlet side, it is controlled differently than if it has a radiator steam trap.