Why is this lower concentric knob stuck?
 

The top part used a flathead, it unscrewed and came off easily. The bottom part has a hex screw, which unscrewed easily but the knob won't come off. Is there some trick that I'm missing? I've used borderline ridiculous force, there's nothing I can see inside the control cavity, and there is a little tiny circular indentation on the top but I don't think it's possible that it is related...

Dave,

My concentrics are flat head, top and bottom. Think it could be rounded out?

Best of luck,

Bob

Rounded out? Which part?

Screw head on the lower concentric screw. I'm thinking it's actually not a hex...

May need to be drilled out if that's the case.

All the best,

Bob

Nah, it's a hex, I unscrewed it.