This is basically a means of grinding from the reinforcement content that was added to bolster the buffer tube/extension space, and reprofiling the area to make it resemble the initial A1 location. This could incorporate some welding or filling of some type. Use the pictures down below as an example: https://mylesgcxlt.wikigdia.com/6696400/colt_combat_elite_commander_45_acp_for_dummies