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If,,,,and that's an if,,,, it's a Norton or relative of one, ,,,then I would take a penny & place it on the opposite corner of the chunk missing(already damaged corner) & lightly press down on it...

the color on mine & photo # 3 was manipulated to help bring out the texture on the inside of the stones.
I don't rely on color that much, especially when different cameras & conditions are present. ...

After looking at photo # 3, I put my money on a Norton India oilstone. Mainly due to the texture of the inside area of the broken section, man made. It's just in rough condition.

Here is your # 3...