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I think you can also add to the list of flat polished bases: the dated spoke designs, which were usually set in clear glass; the scattered millefiori on muslin weights; and the urchins.
When I bought 400 plus Strathearns a few year ago I found that one or two conventional coloured ground spokes had flat polished bases, but the domes were low, and so I think they has deen ground down to remove an excessively large pontil mark or perhaps damage.
Alan