Fowler Certified Space-Blox 53-680-050-0 36 blocks Manufactured to meet Federal Specification ASME B89. 1.9 - 2002 tolerances on length for inch gage blocks. Certificate of Calibration traceable to N.I.S.T. Tolerances to .00005" Ground, hardened and tapped 4 connection screws included Includes shop hardened fitted case