The Silver Series Rockwell(r) hardness testers include four models: the RA-1 analog manual regular-scale tester, the MRA-1 analog motorized regular-scale tester, the 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 can accommodate specimens up to 6.7 in. (170 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 has two diamond penetrators). To handle odd-shaped parts, the testers have removable, spring-loaded specimen clamps.