This Is basically a means of grinding off the reinforcement material which was included to fortify the buffer tube/extension region, and reprofiling the area to really make it resemble the first A1 space. This could contain some welding or filling of some kind. Use the pictures down below for example: https://spencerwtybi.wikiworldstock.com/1082443/not_known_facts_about_colt_combat_unit_rail_gun_45