The ASA should be held secure on the rail by a single set screw. The set screw is located in the grip frame just under where that battery sits. Line up your ASA where you like it and tighten this screw. It will press against the ASA securing it. Hand tighten only do not use too much force or risk stripping the screw.
All It Really Takes...Is One Shot To Get Them Out....Then 5 More To Let The Ref Know