Why is there a gap in the seat (between the legs)?
What function does it have?
I can't speak to all aircraft, but on the 727-100 and 747-100/200 aircraft I flew, the seat cutout accommodated the strap on 5-point seat belt systems that went down between your legs. See
I don't recall for certain whether the control yoke could be brought all the way back into the cutout, but I don't think it could. I may be misremembering though.
As I remember, the camlock into which the other 4 straps (left and right across the seat and left and right shoulders) was permanently attached to the strap going down between the legs. On sitting in the seat, you reached down and pulled that strap up with the camlock on the end and then put the other 4 straps into their slots.
I greatly disliked the shoulder restraints, so once we reached cruise climb, I would rotate the camlock while holding the left and right seats straps in. This would release the shoulder straps.