User blog comment:B214/Question and Answers 6/@comment-12.188.116.56-20190716202632/@comment-10330822-20190717100827

DevilSlayer is right, the key to Euclid making the replica was having the corpse of Issei's previous body. He couldn't have made it without it, its not that Euclid had discovered some secret new way to develop Longinus-class artificial Sacred Gears.