I can't figure out how to remove this shower knob. I have done knob removal before, but this one is quite perplexing. It seems to have a metal cap that I would think would be taken off to reveal a screw. However, I can't get the metal cap off. I have tried hammering a small screwdriver to get under it, but it doesn't seem to separate at all. There is no screwhole or allen wrench hole in the side or anything like that. Any ideas?