ORAC I went back to that site, and bolstered by your post, I looked further than the
Locating controls section - which does
not have that button listed.

But further down,
Using etc. clearly states what the invisible button does. Double
My new NR498E is actually quite nice - apart from the keyboard. The screen quality is outstanding for that price range. ( I paid $645 - on sale but with normal guarantees ) The 510 ER seems to have that little bit more build quality and a much more pleasant keyb.
As for the reason they didn't send it back...the plot thickens.