This "Ansichts Karte" or 'viewing postcard' (a term the Affair enjoys), sent in by Norbert, explains some of the mechanics of "Post Kunst" or 'Mail Art'. It is #1312 - that is a long series (a fierce campaign?).
A sharp looking Koczorski Artistamp up in the corner - nice work! Thank you very much Norbert. I shall be making a return Ansichts Karte to Germany very soon!