Set made of fully seasoned alloy tool steel with hardened to 65 RC to provide both long wear and resistance to stains and rust Blocks manufactured under carefully controlled conditions, allowing for the highest degree of accuracy when used at room temperatures of 68 degrees F (20 degrees C). All gage block sets are certified by an accredited ISO-17025 gage lab with long form certification Individual serial numbers on each block, regardless of grade. A complete certificate of calibration traceable to the N.I.S.T. showing the deviation of each block. All sets meet or exceed Fed. Spec. ASME B89.1.9-2002 Grade : AS-0 Mirror polished surfaces provide superior wringing characteristics. Guaranteed Accuracy of Flatness, Squareness, and Parallelism Excellent Finish and Wringing Characteristics All sets are supplied in deluxe fitted wood case Grade of Gage Block Set: ( AS-0) Blocks are intended to be used within an inspection area to verify the accuracy of plug and snap gages as well as for setting electronic measuring devices