I've been offered a surface grinder

[Vinny]

That letter is the grade or hardness of the wheel. They run the alphabet from C-Z, C being the softest and Z the hardest. Full explanation is at the bottom of page 4 and the top of page 5 in "How To Select The Right Grinding Wheel" from Norton. Sunset posted it in my thread on the Boyer-Shultz. If this link works, here it is: http://www.metalworkingfun.com/attachment.php?aid=12193