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I don't recall ever seeing a Strathearn 'millefiori on muslin' that had anything other than a flat polished base - I've never seen one with a fire polished 'dimple', like you find on most Strathearn spokes. The clear ground Strathearn spokes - often dated - had flat polished bases, as otherwise you would see the uneven base from above. I think that is the reason why many of the clear ground 1930s Paul Ysart weights have flat bases - so there is no pontil mark visible - but.... the flat polished base can get scratched. The antique French / English / American weights usually had a polished concave base if they had a clear ground, so that scratches would not disfigure the design when seen from above - they would just be on the wear rim, which is not visible from above because of the optical properties of the design.
Alan