I've just checked my dad's A V Roe Instructional Course Handbook (JUL '45) on the Type and it gives no clues to the Window chute. It does label the protrusion shown in photograph posted by
forget as a ventilation extractor vent, though. It's not as clear as in the manual but it is marked on this Flight drawing;
1943 | 2865 | Flight Archive
In The Lancaster Story by P Jacobs, he notes that the flare chute was used for Window up until early '44. The flare chute was aft of the ventral gun location and by the entrance door.