IMO, the Axe Mortalis is simply a Master-Crafted Power Axe. I feel that this entire debate has been ignoring the fact that 'master-crafted' is not a unique special rule, but rather an upgrade to weaponry (as evidenced in other portions of the codex). That, and going by the clarification in the BRB to look to the model for clarification as to whether it's a Sword/Axe/etc. seems that the debate would be easily resolved. Dante carries a big-ol' power axe with a unique name, which has the master-crafted upgrade.