The belt drive SB that I recall seemed to have two clutches, one for everything and one specifically for the rear roller. The everything clutch consisted of tightening the belt with a hand lever.
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Does your belt have an automatic tensioner, or a belt clutch? As I recall you have to route the belt to the rear of the tensioner, or it can't work.

Incidentally, this is the picture that shows what happens if you don't glue the clutch facing material to the disk.