I'd expect a few variations on Lancaster Window chutes. Here's what I thought was the final design, just above the letter M.
Propellor Flying
Tilt.
That's an exhaust vent for the cabin heating. I've got a sectioned diagram of a Lancaster which shows 'Flare Chute' below the Mid Upper Turret. However, I do know that some window chutes were fitted well forward in the nose.