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I inherited what appears to be an Atoma Diamond Plate.  There are no markings on the plate, but its measurements are identical to Atoma's plates.  The surface has that pockmarked appearance of dots.  The backing plate is aluminum. 

My problem is, I do not know the grit:  140x, 400x, or 1200x.  Would anyone have an idea how to determine this?

On the Lee Valley site, they show a ruler placed on top of each plate, so that you can determine the grit. Here's the pic of 140, 400, and 1200 grit plates.  The link is also provided.

http://www.leevalley.com/us/wood/page.as...at=1,43072

Thanks.  Very helpful.  I think mine is 1200x.

Greg