In 1996 a limited number of Catan base sets with the yellow Kosmos logo featured a wood hex with text on the back. This text promoted the newly released 5/6 player set to expand the game for more players. I have only seen this is the yellow 1996 sets and it does not appear in any of my later yellow logo sets, nor in the later blue Kosmos sets.
The text read "You want to play "The Settlers of Catan" with five or six? No problem: the supplementary set provides the additional material. You can find it anywhere that there are good games" I think this was a smart idea and in theory cost nothing extra as they were already printing the letters on the back of the number counters on the same sheet.
This is the only example I am aware of where a hex that was part of the game was used to promote other Catan products. The other good example of this would be the Catan Event Cards from 2005 that included extra cards promoting other Catan games and expansions.
