Those are insert steels for a die set like others have said. Dies and molds are made in sections to make them easier to fabricate, repair, and make spare parts for as all tools, even 60-62 rc d2 wear over time. Especially in high volume operations, (think printing, fastener production, etc, thousands of parts er day). Those are component steels that are part of a larger tool, and the detail number is stamped into the machined area so the surface is still flat and the parts are identified. You probably can't use them for anything other than a heavy block of steel. They may crack if you weld with them, or impact them in any way, like with a hammer. Even dropping them can make them shatter. They're hard enough that nothing in your garage is going to work them effectively, even a grinding wheel would damage the surface, but it would be sloooooow cutting. Hand stoning and finishing of an edm cut steel is really the only way to work those back, and it takes a long time. They're really cool, but for most folks, just really heavy and super hard.
