Hi Karol. What you have circled on your first drawing is one wall of the air vent up from the boiler room. The solid line to the left of it is the air screen. The dashed line to the left of that is the actual uptake. On most drawings the air screen is more prominent than the uptake.
At one point in Mr. Pounder's pg19 drawing, what he has marked as an air screen is just one wall of an air vent.
I think the various parts we are looking at are: -The boiler room casing is part of the ship's structure. It starts from the outer funnel and is formed by the bulkheads from the fiddley top down to E deck. That deck, the bulkheads outside of the fan rooms, F deck, and the hull complete the enclosing of the boiler room. -Against the fore and aft walls of the casing are the draft air ducts down to the fan rooms and the vent air ducts back up. The draft duct is formed by the casing down to E deck and a divider down to F deck. The vent air duct is formed by that same divider and the divider you pointed out starting up from G deck. As you say it shows on the side to side cross section drawings but not on the fore-aft drawings so I am not sure how it runs. There are walkways and pipes that might interfere with it. It may also be part of that extra wall you pointed out on the side bunkers. -The uptake starts from the boiler front and runs in a streamlined shape all the way up the funnel. -The air screen closely encases the uptake from the top of the cleanout doors on the boiler front up to the fiddley top.
Probably what we need here is an isometric drawing to sort the shape of the vent air duct at G deck.