Xray NT1 progressive springs
From Xray for the NT1 these new progressive springs are soft at the beginning of the compression but become harder as the spring is compressed more in the middle, with the hardest at the end. Pearl-grey color and laser engraved for easy identification they are available as a pair, with the springs individually measured, controlled, inspected and matched to provide identical damping characteristics. During the selection process, each spring is compressed in several fine increments and the values are recorded. Each spring is then matched with another spring of identical characteristics.
Source: Xray [teamxray.com]