...Narrow Chisel Or (Not Your Best) Screwdriver, to fit inside the Slot, You need to get the 'Pry' Underneath the Indent, and Hit it with a Ball-Peen Hammer. When Aligned and reassembling...just tap it back into place, using a DULL Punch. I know most will use Two 'Pipe-Wrenchs' to Un-Screw the Collar. OR, To Hold One End….Chuck it up in 3-Jaw Lathe Chuck (Slide ROD Into Spindle) and LOCK the Spindle from turning, And Try a 'Strap-Wrench'. They use these on 'Rifle Barrels' because they DO NOT Mar the finish. If that doesn't Grip, You'll need the Pipe-Wrench.