It's supposed to be 100% basswood ply (Adam has asked the question of the factory and that was their reply), though I have my doubts. It looks more like a birch layer to me (if it's not maple). It also feels tougher than basswood. It's probably a thicker layer than the standard PBG veneers, but maybe only 1mm thick, so it's still possible to sand through. But being a ply I think the glue used is different to and stronger than the veneer glue, so less danger of it coming loose if you use a water-based stain on it.