This is largely a process of grinding from the reinforcement material which was additional to improve the buffer tube/extension area, and reprofiling the area to really make it resemble the original A1 location. This might consist of some welding or filling of some sort. Use the images under for example: https://coltcombatelitedefender01111.yourkwikimage.com/1070362/gold_cup_trophy_colt_45_an_overview