This is largely a strategy of grinding off the reinforcement content which was included to strengthen the buffer tube/extension region, and reprofiling the area to make it resemble the first A1 place. This will likely include some welding or filling of some sort. Use the pictures underneath as an example: https://gold-cup-trophy-colt-4501111.wikissl.com/1080423/indicators_on_gold_cup_trophy_colt_45_you_should_know