[gpvos]

That's RISC OS 3, which is several years later. Arthur already had the X in 1987, see for example http://mobile.osnews.com/story.php/18941/mobile-opt-out.php . (Later than the 1985 find in Atari OS of the original article, but still interesting.)

As far as I know, Acorn's RISC OS also pioneered the icon bar and the context menu.

[LeoPanthera]

I see that, but it's the same symbol as pre-NewLook RO3, and is only charitably described as an "X". It's more of a splat.