THIS IS TO ALLOW FOR A RMR OR OTHER RED DOT SIGHT TO BE RECESSED INTO THE SLIDE. THIS CUT ALLOWS FOR THE SIGHT TO BE CLOSER TO THE BORE AXIS. THE CUTS ARE DONE IN HOUSE WITH A 3 TO 5 BUISNESS DAY TURN AROUND. WE SELL RMRs AND SLIDES IF YOU NEED ONE OR YOU MUST SEND IN THE SIGHT AND SLIDE FOR FITMENT....