If you get the iron plate, it can easily be made flat, and providing it was properly manufactured so that it is stress free it won't warp or move. Unless temp differences are introduced. Even so, once temp is equalized, the plate will return to shape. You are still left with the need to qualify it's degree of flatness though. A granite stone of known quality can be used for this if used to show bearing and scraping the plate using it for reference. It's more expensive to repair surface would be preserved if used very sparingly, such as to bring the iron plate into shape for use during scraping, and setting up on for inspecting parts and tools. A trio of iron plates of the same size can be marked against each other and scraped as a set until all three are flat to the degree you need or are capable of making happen. I would probably get the iron plate knowing I would need to get something to check it against before I could use it. I couldn't resist buying it at a deal though. As to Bill's observation, I wonder how the iron plates he used for marking bearing were scraped flat. What method is used to check the plates if not against a stone? Is the three plate method commonly used in scraping to preserve your granite surface? I'm always up for adding tricks to my mental toolbox
) Ever curious.
