These are the rollers that fit in the cage at the rear of the inner clutch body - there are 15 rollers fitted but we include a spare in case you drop one!, so 15+1=16 rollers.
They support the clutch basket as it spins around the body. These often have slight wear as they take a lot of thrust and load from the engine. They can be accessed by removing the 3 nuts at the back of the clutch body, which are normally centre punched to stop them loosening. Once the rollers are replaced, retighten the nuts and centre punch them again. By the way, If converting a normal cluthch to a Manx type, you will need to machine the body to reduce its width, in which case these rollers should be replaced as a matter of course