Hand Tools Archive
I think I have what they call fine (marked 600 grit) and extra fine (was marked 1200 grit, but I lost the sticker).
They were both brash when new but are now a lot finer than their initial use after knocking off outliers. I haven't seen finer from them, but no problem as the cost of the 1 micron diamonds now is something like a quarter per carat in quantity. I'm not sure $25 worth of 1 micron diamonds could be used in two decades of full time work.
I like them more because they've settled in fine. The 600 is similar to a 1000 grit waterstone and subjectively, the 1200 settles in close to what a 2k grit stone is.
If you got your plate a long time ago, the composition might be different - mine are a combination of monocrystalline and polycrystalline diamonds.
My pair are not totally flat to each other, but they're close.