What are these openings on the TBD/TBM Avenger?
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.