You have prompted me to examine mine and from the base rim it also appears to be 3 layers !
(http://glassgallery.yobunny.org.uk/albums/userpics/10006/1903%20007-2.jpg)
Oh, was just plodding through Miller's
Collecting The 1960s and there's a photograph of your lamp in yellow (yours looks lime green on my monitor).
It's Italian, unknown manufacturer, but is apparently
borrows liberally from a design by Italian architect Tobia Scarpa for Flos. Estimate: £120-150.
