This Is basically a means of grinding from the reinforcement material that was added to strengthen the buffer tube/extension area, and reprofiling the region to make it resemble the first A1 place. This will likely incorporate some welding or filling of some kind. Use the pictures down below for example: https://coltcombatelitedefender68122.wikicorrespondent.com/5877455/the_5_second_trick_for_colt_combat_elite_commander_45_acp