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I've seen guys refer to the slurry on the BBW side as purple. It seems to me that the ones I have are reddish brown, to brown. Sort of a chocolate on some of them. The slurry might have a purple...

It appears to be a natural (not glued) coticule. The yellow side is so fragile that they used to glue BBW to the back of it to reinforce the yellow. In later years they began using slate for that...