The second dark and square plate looks like a machinists' granite surface plate. They come in 2" and 3" thickness and a variety of sizes. I believe machinists use them to calibrate tools because the granite is exceptionally level. If you're in the US, you can get them for a reasonable price at the global behemoth based in Seattle.
The second dark and square plate looks like a machinists' granite surface plate. They come in 2" and 3" thickness and a variety of sizes. I believe machinists use them to calibrate tools because the granite is exceptionally level. If you're in the US, you can get them for a reasonable price at the global behemoth based in Seattle.