Anne, I have four small Duchess pot lids in clear, the measurement of all four from underneath (the outer edge of the inner rim) is 82mm, and I have two large pot lids in clear, which the same measurement is 97mm.
They are all what I would consider very good quality, and what I presume to be highly polished bases.
The only difference I can notice is that some are a more greyish colour, and some more creamy in colour.
Also I have noticed that the Bagley State tray and Libochovice equivalent seem to have different tray patterns, and the Libochovice tray has feet, where the Bagley one doesn't.