My pleasure Sidnik, ask Google for a cross section of a scotch marine boiler and you should find all you need.
The principles of design have not changed very much today save for welding replacing riveting , bolted stays , and assisting tube tightness etc . Stress relieving overcoming all the negatives of these methods.
The two pass design has been superseded by a three pass , one pas down the furnace, one through the tubes to the front and the last pass back to the boiler rear to exit . The smoke goes up the chimney just the same!