Ah yes indeed, I had the F-105W. I thought the F-91W also had this EL film, because it's also labeled "Illuminator" on the big picture at the top of the article, which was casio's branding for this technology at the time. But it doesn't have the blue branding. And I think that pic at the top isn't actually an F-91W.

The EL light was super, very bright and very well distributed (because the whole backlight film would light up). I'm surprised they managed to put one in such a small watch as EL film needs fairly high voltages, you could hear the switching coil whine when it was on :P Yet it still lasted a good while on battery.