The first photo below shows where the four-ringed superheater inlet dips below the boiler top surface.
Eagle eyed observers will of course notice that the first photo shows the superheater's inlet and the second shows the superheater's outlet. The inlet steady plate has still to be done. The three parts are not easy to make and have to be shaped carefully for a snug fit.
The gaps have to be well sealed in the same way that a conventional steam loco has to have a sealed smoke-box door. In either case, if there are leaks, when the loco chuffs, it draws air in through the gaps instead of up through the fire. The consequence being that it will lose steam generating efficiency.