Four colour proofs in trichromy. To the naked eye they're nothing but dirty screening. Each gel kills the inks of its own colour and turns its complementary black: the red gel kills magenta and yellow and turns cyan black.
One of the four carries two digits, one in each ink. The one that counts is in the same ink as the registration cross in the margin.
The engraver hid the combination to his strongbox where nobody looks: in four colour proofs stacked with the rest. Each digit is printed in a single ink and camouflaged among screening of the same colour. To the naked eye there's nothing there. With the right gel on top, the ink turns black and the digit jumps out.
—