"M" bolt head

[breakeyp]

2. "Face of Bolt Head and Hook only to be case-hardened to a Minimum depth of .01" "

How do you measure this in production? Case depth requires the part to be cut in two, polished and dye etched and inspected with a microscope.

By today's standards, the specifications and materials listed are all over the place. I suspect heat treat performance between wrought iron and mild steel is very different. It took true craftsmen to main the quality that they did.