Hi, Myfav. Those GeoPuzzles are great, aren't they?
Doing jigsaw puzzles from the backs of the pieces is not extremely uncommon in my experience, but not average behavior either. Neither of my children has decided on their own to do puzzles with the pieces flipped, but neither of them showed a very great interest in jigsaws either compared to other types of puzzles and models.
The behavior you describe might be a manifestation of a photographic memory if your son instantly grabbed each piece, regardless of orientation, and placed it correctly, thus doing the puzzle quickly and without any problem-solving. On the other hand, actually working (problem-solving) a jigsaw with country-shaped pieces this way would be easier than a jigsaw with regularly shaped pieces. And even for someone with an ordinary memory who has begun learning geography, the shapes of many of the countries will be distinctive and memorable early on.
Thus I think that unless you've seen other evidence of an eidetic memory (reciting any text verbatim after a single hearing or reading, etc.), it's not likely a sign of such a memory, which after all is very rare. Instead it's more likely to show some combination of above-average memory combined with good visual-spatial skills.
Both of my children seem to have memory more like mine-- they can remember the meaning or effect of large amounts of things, but tend not to remember words from stories, etc. verbatim.
