They are a BS 2470:1973. The technical data for the material is that it is high tensile grade alloy steel, heat treated to RC39-43. The material has a tensile strength of 190.000 ibf/in2 to 180.000 ibf/in2, a yield strength of 170.000 ibf/in2 to 162.000 ibf/in2, and a shear strength of 114.000 ibf/in2 to 108.000 ibf/in2.
eBay