I've been looking at some pictures and models of the TBF1 and TBM3 Avengers, a torpedo and dive bomber from WW2. I've noticed in some cases there are these openings/slots towards the leading edge of the wing on the top, and it seems as if they pass through to an opening to either the very leading edge of the wing, or just below it. The strange thing is that I've seen many pictures and have noticed that in most models these openings/slots are absent. I'm pretty sure they're more common on the TBM3 than the TBD1, if at all. I was wondering what function these have. I have seen one 3D computer model that shows it's just essentially a rectangularly shaped hole going through the entire wing, though I'm not sure if the modeler just guessed at this. Any ideas of what this might be? You might need to zoom in to see them better.
It's a leading edge slot used to improve low speed handling.
Excerpt from Wikipedia:
Partial-span slots are usually found only on the outboard portion of the wing where they ensure airflow over that portion of the wing will remain unstalled at higher angles of attack than the inboard portions of the wing. This ensures the wing root stalls first and contributes to docile stall behaviour and maintaining aileron control throughout the stall.
You'll notice that the slot is directly in front of the ailerons. Since the TBMs landed on aircraft carriers very close to a stall, the slot ensured adequate roll control during approach and landing.