The theme was inventions. Only two countries, Gibraltar and Iceland) used the four CEPT letters instead of the official logo. Most stamps issued in late April or around the 3rd May.
Andorra
Austria
Azores
Belgium
Cyprus
Denmark
Faroes
Finland
France
West Germany > Germany
Gibraltar
Great Britain
Greece
Iceland
Ireland
Isle of Man
Italy
Madeira
Netherlands
Portugal
San Marino
Spain
Sweden
Switzerland
Turkey
Turkish Republic of Northern Cyprus
Yugoslavia