Rail slides off! Why?

Well in that pic its not all the way on.

 

I'm not even sure that style rail will clear the rear sight you have there. Looks like a yugo style rear sight on the dustcover.

 

If it will clear you may just need to adjust the nut on the bottom of the mount (directly at the bottom of the shaft that runs down from the lever) so it will clamp down tighter.

ah seen ya thread, looks like you got it figured out