Exactly NSK. It seems that the centers from the 1903 issue don't fade, the used dark one on my side is wmk2 and I soaked it, no changes. It would be to confirm that mint sample with wmk 3 has dark center, or if mint ones are already paler too.
I only have WW Scotts catalog, they speaks about varieties in specialized catalog, I wonder what are those varieties.
Sorry I need to edit previous message. The color of the vignette is water-soluble in both issues :

Stamp is 'nicer' now but well, maybe not the best move from me!