Stilnovo Gae Aulenti repair issue  

Hi  there,

I need to repair my Stilnovo Gae Aulenti lamp (At least I think it is this one). Mine is a bit different from the ones I find on the internet: I have to attach them to a rail to the ceiling. I actually also have the set of speakers that are part of it.

Anyway. To repair the lamp I have to take of the knob you see circled in red in the photo. Is there anyone out there who can tell me how to do that? I don't want to force things... (I already took off the screw there)

I hope the image is self-explanatory. If not, please send me a message. I would be very grateful if someone can help me or forward me to someone who might.

Kind regards,

Floris