Dave - you cannot visually distinguish a current production FatBoy from an AXY (so stop staring :-). However, it occurs to me that you should easily be able to detect a weight difference, if you're willing to go to the effort of pulling them out and putting them on a scale, due to the much larger magnet in the FatBoy.
I really think it would be beneficial if Alembic made it a little easier, perhaps simply by scratching something into the back side while the potting material was still soft (they mold the shell, stick in the magnet/coil, then fill it). I happen to have a custom AXY that is personally engraved by Mica in just this way.
Or perhaps they could publish the average weights, for both the 4 and 5/6 string versions, so that if you only have one in hand you have a good chance of figuring out what it is.
For anyone else interested in FatBoy vs. AXY comparisons, Joey has started a discussion over
here. (Just skip right over most of the first sentence...)
Hey Joey, have you got a scale?
-Bob