Radial Four Valve Chamber
RFVC motors have a separate cam and cam sprocket setup.
On older XR's with RFVC motors have a cam with the threads for the sprocket mounting bolts as part of the cam. The little flanges that stick out on the end of the cam.
On newer RFVC motors, these flanges with the thread holes are removable, prob because people kept stripping the threads of those cam sprocket bolts and ended having to replace the entire cam.