My theory is... the MDR-CD900CBS is the one model designated for CBS-Sony Music Japan recording studios use only... and they sell the MDR-CD900ST as a special order consumer model... I don't know, just a thought... (I say the Japan studio cuz of their page here:
http://www.so-net.ne.jp/Shop/phone/ and I've seen one Celine Dion video where she uses the 7506 in the Sony Music USA studio... forgot which song tho, probably "Tell Him")...
edit: Another theory, Sony dropped the CBS name in favor of Sony Music with their recording studios and hence the model designation change to ST meaning Studio.
No it's not the Japanese MDR-7506, since in Japan you can buy the 7506 for around 2000yen cheaper than the CD900ST... the 7506 and V6 use 40mm PET diaphragms while the CD900 and V7 use 50mm SED diaphragms...
I paid 15 bucks CDN and ordered the 7506 / V6 hanger just to find out that it cannot be fitted onto my V7, and spent 15 bucks more just to get the V7 / CD900 hanger... sigh! So don't be fooled by their similar appearances...
But from old usenet threads I've read on Google, they all sound very very similar tho...