I took this pic from the Internet to illustrate what's going on here. This is the center of the Mayan calender - yeah, the so-called "Doomsday Calender" of popular hyperbole. First of all, nothing implies doom here - where did that notion come from? It sure caught on! Secondly, it looks very similar to the Mail Artist George Maciunas developed head in the first panel. See Uli's explanation below.
Side B of Uli's card boasts a pretty cool "Par Caravane" (by Caravan) sticker. Nice. Also, Uli's thoughts on the 'face'; of which he was led to believe was Aztec in origin. So, I checked for Aztec head imagery and all I could come up with was this Aztec 'greeting' head (see below). Well folks, now you know what I know - you make the call. Anyway, thank you very much Uli, for the postcard and the info to the puzzling evidence. And oh, I nearly failed to mention the background of the card is a map of Centralia! Uli has cleverly placed the destination address next to the 'A' designation, where I am as I write this. To the left, you can also see the confluence of the two rivers Skookumchuck and Chehalis. That area is curiously fascinating for some reason. Guess you would have to see it. Anyway, I shall see you soon in the post my friend.