They are De La Rue colour comparison stamps. As far as I am aware, not the correct comparison by the seller in the link.
They 'escaped' from the Dutch printers Enschede and were first seen being sold illegally in an Amsterdam market. This is the information I was given and I have not been able to confirm it.
Then I read that
ebay had banned their sales as a few appeared online [pressure from higher up maybe?].
They should not be in private hands.......but then neither should Forgeries and other such peripheral items.
Londonbus1