Have a look at this article on a famed example of the 247.
The image at the top is the aircraft during its flights with the RAF, likely some time in 1944. Note the reverse-slope windows that are a hallmark of the 247.
Now look at the other aircraft in the same article. They have conventional style slope-down windows. Those two images are from before 1944, one in the 30s and the second around 41/42.
Does anyone know what's going on here? Were certain batches one way or the other?