What I can get done cheaply is with 4020 quartz. (The bag says 20% of the grit doesn't pass through a 40 grit mesh) Checking with McMaster - that seems to be about as coarse as it gets.
I'll see if I can find a scrap of half-hard C110 and send you a sample. If I can't find some C110, I can send you a sample made from some ultra pure, dead-soft copper sheet, but I don't know how representative that would be. (I suppose if the dead-soft stuff isn't eroded too excessively, the half-hard C110 certainly won't be.)
Anyway, I'll see what I can come up with.
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