Don't know these very well, but if there's a clutch it'll be obvious (nylon/delrin pulley and it's sort of spring loaded on the brass clutch-plate (if the 160 series and siblings are anything to go by).
I just ought to ask if the belt is actually a proper Thorens one? Some of the cheap equivalents look the same, but use a much harder rubber compound. Official belts have 'Thorens' printed on the outer face.
In the case of the TD150 and 125mk1, the belt had to slide on the fixed metallic pulley and only dropped off if old and stretched (you can't change to 45 either when the belt is too stretched).
I'm so sorry, I can't offer any more. Failing everything, a dusting of talc on the belt may well help rather than hinder here. Check with UKD, who I think are still the UK agents for the brand
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me