There is a small allen head screw in under the spring....remove it. You may have to press on the tailcap while unscrewing to help it not twist. After that, pull the guts out. Next take a small pair of needle nose pliers and remove UNSCREW the black plastic disc.
It should come apart after that.
Post a picture of the side view the inside down view and the back view, then we will know for sure, light it up extra well with a flashlight for kicks.
Bueller?.....Bueller?
C'mon, anyone?
Surely someone has replaced the rubber boot with one of the colored ones from DX/Kai? That's what I'm trying to do. Does that help?