Here`s a pic of a repro and an original lever without the slide extention.
This is just my opinion but I refuse to believe that the extention slide doesn`t have another part. I think that the slot on the shaft connected to a wedge nut, kinda like how a bicycle handle bar is slid into the frame and locks into place. to loosen it will turn one way, to tighten it will turn the other way. If you look at the bottom of the original lever on the left, the crimp is such as to not allow it to fall out the bottom should they separate. Just like a bike frame. The repro doesn`t have the same crimp at the bottom.
I don`t think the slot allows for pinch fitting !i