So here's a little mystery which goes back to 2006.
On the left, the original version with colours as listed in the catalogue: blue-green, greenish-yellow, silver & black.
On the right, the 2006 version with the yellow replaced by black - but the stamp looks the same. Indeed, the stamp IS the same, I'm sure yellow is still present (* no it isn't!). Below, the date and colour-check marks in the right margin, also with the yellow replaced by black.
So what's the point? If the yellow wasn't clear enough to see when the sheets were checked, how does replacing the yellow cylinder number with a black one - which must have been printed with the black cylinder - help anyone?? * Official response here.