Andy, I just spent all afternoon removing the same knob. I had put a new set screw in last year because the original got stripped and kept sliding off the metal bar. The replacement rusted just like yours. I tried everything and finally used a hack saw to cut the knob in two. Unfortunately, the skeg wire looks like it is fused to the slide bar and I could not remove the skeg wire. I disconnected the wire from the skeg and taped the skeg up into the skeg box until I have a repair done. I think I'll have to get Carl or Tom B. To fix it. While working on the slider, I discovered about 4 "holes" just under the cockpit combing. I think the resulted from when the boat fell off the hullavator a couple of years back.
Anyway, in answer to your question I got a new knob from Greg Paquin last year. I should have installed it then.
Dave