Britain's most spectacular commemoratives, No17: 1997 British Aircraft Designers 20p

If the picture concealed in the hide of the cow in the Patients’ Tale stamp (see No23) was subtle, the optical illusion here is even more so.

The eye is drawn by the iconic image of the Supermarine Spitfire banking against a cloudy sky. But look closer and you realise that the clouds are not just random shapes, but form the profile of the great aeronautical engineer who designed the plane, Reginald J Mitchell. 

Portraying the vital war machine with its mentor overseeing it paternally from the skies was a clever idea. And it worked better in this design than in the other four in the set.

Design: Turner Duckworth.

Printing: gravure by Harrisons.