For GLOCK 21, you can now mount the RMSc without needing to mill/cut the slide to fit. This mount fits by replacing the rear iron sights, with a slide in rear sight and mount plate. It sits flush with the RMSc and SMSc. This mount will also fit the standard RMS, SMS, SMS2, Jpoint sights but with slight width overhang. The RMSw will fit but may not seal as the plate is narrower than the RMSw and its gasket.