A79 often designates a specific, high-quality Aluminum Oxide (synthetic) sharpening stone, commonly associated with oilstones and manufactured by companies such as Bay State

Sharpening stones labeled with "40" or "C40" typically refer to very coarse, low-grit stones used for heavy-duty metal removal,


I have stones up to 6000 and down to 300, the stones on each side of this are not as coarse as 300 so I'm not certain the C40 really is very coarse at all.  I did clean both sides an it made little difference.  I'll continue to use this stone for old chisels that I use for cleaning etc.

Thank you for the input.