The Silver Series RockwellR hardness testers includes four models: the RA-1 analog manual regular scale tester, the MRA-1 analog motorized regular scale tester, MRD-1 digital regular scale tester and the MTD-2 twin scale digital tester. Regular scale models are designed having throat depths of 6.5 in. (165 mm) and accommodate specimens up to 6.75 in. (171 mm) high, whereas the twin scale MTD-2 has a vertical capacity of 11 in. (280 mm). Each unit is equipped with test blocks and a single diamond penetrator. The MTD-2 twin digital tester includes two diamond penetrators. For odd-shaped components, the series comes equipped with removable spring-loaded specimen clamps.