The "2" stands for Almen gage #2. This is now an obsolete practice as mentioned in the SAE specification J442 and J443. The orginal gage patented by J. O. almen used two knife edges to support the strip during measurement. In November of 1943, General Motors revised the gage design to replace the knife edges with 4-ball support and designated the gage as #2 Almen Gage. Almen's patent then issued in June of 1944. To insure that users would only use the revised gage the peening procedures adopted the "2" after the strip letter, such as C2. SAE recommends that this procedure be ignored and only designate the intensity as .008-.012 C.
