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I never got a call back from the people at the San Francisco Chronicle about the pronunciation of the word spadia, but I found this: "It's a variation on the partial-page ad that wraps around the comics, which is called a spadea pronounced spay-dee-uh."
However, this is the same source which says a spadia is the same as a gatefold, which it is not.
A spadia is a page wrapped around the spine of a periodical so as to appear as a narrow flap or partial page.
As for "spadia," I'd never heard of it either, even though I used to work in advertising and (editorially) in newspapers (and ruined more than a few pairs of slacks at the presses).
FYI, one source claims that that the spadia appeared first about 15 years ago.
I'm wondering if the Greek root of the latter half of that word might also be the origin of our "spadia."