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Hi Phil, The original poster posted in another NG that they had indeed found the same mark, on a _silver_ site. (I've posted the link below showing the unicorn)
I thought this was very interesting, and in fact might help identify other marks on pottery, ceramic or china that do use metal as decoration on the finished product.
Don't know why the OP didn't respond here, but I wanted to share their results with you.
http://www.silvercollecting.com/silvermarksM.html