that what it looks like, look to me like there is a block off wood either side of the donwleg that is slightly thicker than the leg so the foot slides, the bolt must go through and have a slot in the leg, the cut-out U shape is to allow the ply to flex in and clamp tight against the leg
That is the correct answer.
I made two identical cheeks, one with the U-formed cut. I made a slot in the support leg. I glued the two cheeks together with two strips of wood slightly thicker than the leg, put a bolt through it all and so I had a way to adjust the length of the leg.