Regardless of the part numbers the enclosures are slightly differnt, the 595 grille recess goes a bit deeper as well. IIRC the 555/595 driver unit is also used on a HD2x heaphone, the 280Pro if I remember right. This doesn't mean the phones are the exact same.
It's like Intel CPUs, the high end parts that don't cut it in lab testing get badged to a lower model number rather than being binned as they work perfectly fine in that model designation specification. It's a sound business model.
Plus, what have I got to justify? I buy stuff and return them if they're not good enough for me all the time and post my findings here. If I don't like something it gets sent back, this is what I do and I post my thoughts on this forum and some other technical forums and I don't think you quite realise this even though you've been here since 2003.