Most likely you going to need to cut the plastic knob off the bolt as it is seized. I have seen this on several of these over the years where they over tighten themselves and seize. I personally got an old ST285 that did it eve nwith a new knob then I started using anti-seize compound on it which seems to cure the problem.
BTW that know is already worn out and allowing moisture into the area of the bolt and threads.
Black knobs are normal right hand threaded and red knobs are left hand threaded. As I said just plain stick sometimes even when new ones are installed and this one is worn out and probably has lost the seal over the hex head screw (bolt).