Dasqua Superior Quality 2-4-6 Precision Matched Pair Block Set, Heat treated and case hardened to HRC 58 to 62. Hardened and precision ground on all 6 sides of each block. Accuracy with squareness on all sides within .0002-inch per inch. 23 holes consists of 5 tapped holes that are 5/8-inch-11 for clamping and 18 untapped holes. Cross drilling design gives minimum weight and maximum strength.