Yes, there are lots of pre-Caithness period PY weights with a flat, polished base.
Frank's point is valid but it seems that sometimes when a design was set in clear glass, rather than having a coloured ground of some form, the base would not be polished if the pontil area was not easily visible through the design. Perhaps the eBay weight could just as easily have had a non-polished base. I can't prove it, but perhaps unpolished bases on "clear-ground" weights occur more often on concentric designs where there is little space between the rows of canes.