hi there tipsy
I have the plans of the SS CHANNEL QUEEN by P.N. Thomas; I noticed further along on the vessel plan there is a sanitary tank; that is opposite the funnel; and there is a sanitary tank port and starboard. So, I dont think there would be a sanitary tank down aft - but there may be. But I would have thought it would have been a lot larger size.
On the plan further aft of the funnel - there is a galley skylight which is drawn in as a rectangle, similar to that one on the stern of the vessel. Just wondering if that is what that rectangle represents.
The other thought that came to mind is, whilst going through the the book British Ocean Tramps Vol 1 - by P.N. Thomas at the plans of vessels- I note there are several drawings of rectangular shapes on the stern deck, some of them which are marked in as - what look to be like - canvas chart houses (where they put the charts for shelter - if they have to steer from the stern) in an emergency.
The other thing possible would be a meat/vegetable locker - my gut feeling is though it may be a skylight.
John