My brother has a big old round stone, and is in a big metal frame. I'll try to get a photo next time im there.I seem to remember the large diameter whetstone at my school (pupil powered) was about 2' dia.( metric came in some years later!).
I've just done a little exercise on my trusty CAD package and found that the concavity across a 1/4" bevel would have been 0.0007" ie, less than a thou. Distinct?
Brian