Keith I believe that the bases are cased, that is to say they are cased in a very thin casing of the white opaline, and the lids are not.
However ... not in all instances possibly. I think it is possible there are some items which are cased and some not. And this might depend on when (what years/time period) they were made and therefore, who made them at the time.
But this is just what I believe from what I've read.
It is also possible that they just vary depending on the maker of the item - again iirc from what I have read.
m