This is covered in a number of other threads, but basically is as follows...
Pro SD bodies have a wider 'throat' than the Pro or Pro S to accommodate the K/L 500mm APO and the 75mm Shift. These lenses will not fit onto a Pro or a Pro S because the rear of the lens is wider than the throat of the body. However, all older Sekor and Sekor C lenses, with a narrow rear part of the lens will fit onto a Pro SD body. However, there will be a gap between the rear of the lens and the throat of the body. This does not really affect performance, it just makes the lens easier to mount. So, Mamiya supplies a narrow ring that sits around the rear part of the lens barrel that fills this gap. This is usually supplied with a KL lens, either separately or already mounted on the lens. You just pull it on or off when needed. If you have a Pro SD body, make sure the ring is fitted to the back of the lens of your KL or Sekor lenses. If you have a Pro or Pro S body, take it off the lens before mounting or it won't fit. I think that the manual for the lenses may have this the wrong way round, but this is from (unreliable) memory.