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
Note: please check your roller ring carefully before ordering. As I recently found from an owner of an original SOHC Manx - some Manx clutch's were fitted with 20 roller rings. We do not offer this as a seperate listing, but should be able to supply these sets by emailing firstname.lastname@example.org. Expect cost to be pro-rata cost of 15 rollers + extra 5 and a spare