Loosen but do not remove the three setscrews in the focus ring.
Turn out enough so that the focus ring can be removed from the front cell.
Use the largest slotted screwdriver that will fit in the screw holes.
Blade must have a fine enough taper to fit precisely in the screw slots.
Regrind your screwdriver if necessary.
If sealed with lacquer, use lacquer thinner or toluene applied with a toothpick to soften.