It almost certainly IS leather. They will have used thick leather and pressed it into a mould wet, then allowed it to dry. The polished outer surface will be the skin side, and the "woven" looking side will be the fiberous inner surface. If you want the leather soft and flexible, you need to work it with your hands after soaking it in neatsfoot. I would sand the hard seal to fit, then soak, then work, work to make the leather supple. You can also fit the seal hard, and let repeated use of the gun, eventually soften the leather, but that take 10 times longer.