Examine the lock up in the cocking tube/reciever. The metal of the recievers is a bit softer and cocking tube lockups are reinforced to handle the wear. That lockup looks worn just off of square. Make sure it holds or determine if it needs reinforcing. Not a deal breaker in itself but definitely a price negotiating point. (Not a build to use the HK Slap on because that will continue to wear it down) RCM Barrels are marked or at least the ones I own. If a PCS build, call PCS, He will tell you if he did and how to tell) If barrel is unmarked (and not just that the markings were filled with paint then the barrel may be a Century).