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I use a Translucent Arky pocket hone as a rubber on most all my finishers, It is flat and much harder and does not drop any grit.
I am glad your got a great Coticule!
~Richard

In my opinion, the harder stones, by themselves, cut slower as the surface is hard and prevents much grit from being exposed or loosened from the matrix.
JMO, YMMV
~Richard

I am fortunate enough to have an antique glass hard Coticule that displays the same characteristics as yours.

I have used different slurry stones with it and can do as the above post says;
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