What would happen if you took out the fusion drive and, say, tried to use it in an enclosure? Would you see the SSD and the HDD or is the software built into the drive and not OS X?
If you're looking to put it back together, after removing the back panel, watch out for the screws. They all look the same size at a glance, but the two by the vent that are usually longer are a bit shorter than the rest. If you put one of the longer ones in by the vent it will stick out just a little bit. Here's a pic!