According to Virginia Postrel, who posted this on christmas day last year, this ad comes from a 1974 issue of Vogue magazine, which would put it perfectly in the timeframe after American Graffiti where Lucas had started work on his new 'space western'.
Some people don't think this is much more than a coincidence, after all the crawl supposedly comes from Flash Gordon (and as we've previously examined, possibly Forbidden Planet). But it's less the shrinking text (which isn't tilted, by the way), and more the "Far far away in a distant galaxy' bit which makes this an eeiry predecessor to the Star Wars crawl.
It's not exactly easy to find info about this supposed 'luxury fabric from the 21st century', but there is this trademark filing which lists the registration date as 4/6/1976, and the fact that Robert-David Morton lived on a farm which he called 'Jupiter 8', a name he also used for one of his companies (along with Quadraluna). Supposedly he was the first designer to use lycra for eveningwear.
He also had a shop in Manhattan, on W 37th Streeet, which may be significant, given that Lucas spent a lot of time in Manhattan in the early 70s in talks with Universal around the distributing rights for American Graffiti (I know this through personal interviews with Edward Summer, with whom he co-owned a comic book art gallery here). Maybe he spotted an ad in the window of the shop as he was walking around? Maybe someone showed it to him: "This is like that space thing you're working on."
Who knows? It does indeed seem somewhat incredible, but this could very well be genuine.
Whatever the case, a few years later Star Wars itself would become a feature in Vogue.
Update July 2nd, 2013:
A little while back I got in touch with Virginia Postrel who first found the ad, to do what I could to verify its origin, she wrote me back:
It's from August, toward the back of the magazine but I'm afraid I don't have a page number. I'm attaching a photo that gets most of the spread. You can see it's toward the back because of the partial-page ad opposite.
Since I don't have a cover shot I looked up the issue on Ebay and it turns out to be a very famous cover, the first to feature a black model on the cover.
Article about it here.
I read your post and, aside from verifying that it's a real photo from a real bound volume of the magazine in the Santa Monica Public Library, I can't add much. I found the trademark listing too. The fact that the issue was so incredibly famous makes it somewhat more plausible that someone outside of fashion might have seen it.
There you go. Fascinating.