Wouldn't call it flue (since flue gas is generally used a term to define Inert Gas systems based on exhaust gas from the ship's engine on modern oil tankers), but I do suppose you're right. You can see the pressure regulating devices (currently called Pressure-Vacuum valves or in short PV valves) in the lower left corner of the picture I showed the washing lines on. Close to the most left washing line little more aft and closer to the "flue" line, you can see one closer-by and one slightly more aft a little further away from the pipe.
Currently such devices are connected to the inert gas line so that the gas going out of the tank while loading or during voyage, goes into an inert atmosphere and blows out through a vent mast, or, when discharging, inert gas is sucked in to avoid a vaccuum in the tank. Not sure how they did that in those early days, I'd assume they just sucked in air, not sure whether they did that through the same device/pipe or whether they had something seperate for that. The exhaust are probably the devices we're looking at, to the line you designated. I do wonder where that line ends. Must end up in some kind of a vent mast or kingpost. Anyway nice find!
Great to see these awesomely tiny winches. I do recommend to put a limit at size of objects you'll illustrate to the model. When you start digging into such tankers you end up with more and more detail and a very long building time, or you get an inconsistent level of detailing, showing certain tiny details but forgetting/skipping objects that are perhaps same size or even bigger. One of the reasons I consistently avoid showing valves and fire lines etc. on 1/700 ships. The same I would do for example with the steam lines on this ship. These lines probably branch off a lot more than only to the winches and when the wire you use is only slightly off scale it might give an awkward look. That's of course just my opinion. This has become quite the scratchbuild by now
