The Canon CM-V1 Locking EF Mount Kit substitutes the Canon EF mount that comes with your Canon C500 Mark II. The locking mount preserves the same electronic functionality of the standard EF mount but locks your lens into the mount to avoid play.
Cinema lenses, particularly zooms, necessitate that lenses preserve a specific distance from the image plane for best results; any play amongst the lens and mount can cause the lens to suffer from poor optical or machine-driven performance. The locking EF-C mount includes the EF mount electronic contacts and lets you mount practically any Canon EF mount lens, whichever the stills variety or the cinema lens variety, on your camera.
The kit comprises the locking EF mount and a shim kit for exactly fine-tuning the mount on your camera to confirm the right flange distance.