I have a couple of the "Smith's" brand perforated diamond "stones similar to what was pictured (one "medium" - about 350 g and one "fine" - about 700). Bought one at HD and one at Lowes. Do they work to cut hard metal: yes. are they flat: NO. I won't buy any more perforated ones that do not have full solid support throughout the entire stone (the plastic backing doesn't support it well so the center "dishes"). Also, the edges are very rough until they get somewhat worn down by use.
I think this is a case where most inexpensive is not economical, especially for plane blades, etc. I use them to reset bevels for my cambered blades, because they definitely put a camber in a flat surface!
JMTCW
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