There's probably an iron at the bottom of the opening, and then the question is how far in it goes on the wing. 4 cm isn't much, most likely the iron goes in as far as the stone visible there. They usually aren't that stingy. Place a support in the middle of the part that is towards this side of the photo and chisel out at the top edge towards the outside, a narrow groove towards the outside, then you can follow the iron and it stays in place on the rest.