Well this website is showing the picture of it, but you will have to enquire if they are still selling it. It will just slide on and a screw will tighten it up. http://www.trusound.co.uk/P/409/Gemini+TWMT-3601604+Tonearm+Counterweight+
I can't find on the net anyone else selling parts for this model, still.
Answers & Comments
Well this website is showing the picture of it, but you will have to enquire if they are still selling it.
It will just slide on and a screw will tighten it up.
http://www.trusound.co.uk/P/409/Gemini+TWMT-3601604+Tonearm+Counterweight+
I can't find on the net anyone else selling parts for this model, still.