My recollection was that it looked slightly ground at the very top, so as to be able to provide a definitive boundary for the application of the enamel on the rim. Alisa can also correct me, but I believe that the bottom border of the iridescent area starting at the rim and dropping to the upper shoulder has an expertly applied thin gilt line at the bottom of that area. It appeared to provide a transition line, or border, from the iridescent glass to the clear shoulder. The change from iridescent to clear in this area is a slope, allowing for the smooth area where the color transitions. The gilt line appeared to make it look otherwise. The panels have been cut horizontal to the surface in the main body, creating the thickness of the panels. As Alisa said, the panels are quite thick, I would say at least 1/8 inch, and maybe 3/16 inch. There have been large thick areas of class removed from this piece to create the clear areas on the body.
The enamel work on the piece was quite good.
Alisa, Please correct me if my memories of the execution are incorrect.
Hope this made sense.... Having seen it I can say it is a wonderfully executed piece of glass.